Directional Control Valves - Part 2

Spool Selection:

Spools in Directional Control Valves come in various configurations depending on the applications they are required for. The extreme positions usually have the same configuration where P is connected to A and B is connected to T at one extreme and vice versa for the opposite side. Some of the more common spool types are listed below with examples of some of the applications they serve.

Spools for 2-Position Valves:

2-Position spool valves have only one actuator and the spools are usually spring offset. Although they have all the four ports ( P, T, A and B) on their interface, they may or may not necessarily use all of these ports. All 2-position spools can either be normally-open or normally-closed.

This is a misnomer, however. It is not necessary for the valve to be “closed” or “open” it may just be redirecting flow to a different direction. It simply means that the solenoid and the spring switch positions so the neutral position changes.

On-Off Spool: 

 On-Off spools are the most basic of all spools. They allow flow to pass in one position and restrict flow path in the second position. If the neutral position allows for flow, the valve is called “normally-open” and if flow is restricted in the neutral position, the valve is “normally closed”. The valve is most commonly used for switching on or switching off flows to different sub circuits or as a main on-off valve. It can also be used as a manual drain valve to unload pumps or accumulators. It should be noted that since the valve is of the spool type, it may have certain leakages. For simple on-off valves, it is more plausible to use Solenoid Poppet valves.

Selector Spools (Exhaust Spool): 

Selector Spools are used when one inlet has to feed two outlets depending upon the actuation. Depending upon the connections made to the various ports of the valves, they are also called Exhaust Spools. Exhaust spools are used with Pressure Gauges or Hydraulic Clutches or Parking Brakes or any other accessory that needs to be referenced to tank when not in use. For a normally closed valve, when actuated, the pressure line is connected to the accessory at the outlet; let us say it is a pressure gauge, which gives us the pressure reading. On releasing the actuator, the pressure gauge is reference back to tank, which is connected to the third port, and hence shows zero pressure. For a normally open valve, in the neutral position, the valve is referenced to tank, which, in the case of Parking Brakes, keeps them engaged. Using a hydraulic pressure line as an actuator, the pressure in the two lines of the motor is sensed and opens the valve allowing the pressure line to be connected to the parking brake cylinder, releasing it.

Parallel and Cross Spool: 

Parallel and Cross spools are spools that connect P to A and B to T in the parallel position and vice versa in the cross position. Parallel and cross- positions are more commonly the extreme positions in 3 position valves. These are versatile spools that can be used for any of the applications above simply by blocking the port not required on the subplate.

Spools for 3-Position Valves:

3-Position spool valves have two actuators and the spools are usually spring centered (although they can be detented). They usually use all the four ports ( P, T, A and B) on their interface.

Closed Spool (Cylinder Spool): 

Closed spools have their ports isolated from each other in the central position. The lands are fairly large which keeps leakages down to a minimum. This spool is also the easiest to manufacture. These valves are not to be used as load holding valves in the puritan sense since a certain amount of leakage can expected in this configuration (5 to 10 cc/ min from A and B to T). The transition positions can be either closed as well or completely open.

In the central position, pump flow has no recourse to the tank line due to the blocked position. Hence the pump either needs to be pressure-compensated where the system pressure is at the pump compensator setting until all pump flow is going to the actuators at their working pressures or unloaded to tank either with a relief valve which will relieve it at the set pressure or an unloading valve which unloads the valve at a minimum pressure and conserves power and electricity. The spool is used in cylinders which cannot be drained to tank and need to be kept pressurized (but only for a short while) and the P port is needed in a secondary system to perform other operations.

Float Spool (Motor Spool): 

Float spool valves have their A and B ports connected to T in the central position. Again in this spool, the lands are sufficiently long to prevent massive leakages between ports. The valves are used to relieve pressures at the A and B ports while keeping the P port isolated in order to service a secondary system or be separately unloaded to tank. The valve is commonly used with hydraulic motors (hence the colloquial term “motor spool”). This is because when the oil supply to motor is cut off, the momentum of the motor keeps it rotating. Oil is picked up from the inlet port due to rotational inertia and is deposited to the outlet port. Hence, instead of the system driving the motor, the motor drives the system. 

Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Another common use of this spool is with single or double Pilot Operated Check Valves or Overcentre Valves. These valves, which serve as means to lock cylinders in place, need their downstream ports connected to tank. For Pilot Check Valves this is essential since any pressure entrained in the line may serve as a pilot pressure and open the pilot check valve in the opposite line. 

For Overcentre Valves, they act as load holding valves as well as thermal relief valves. Not only can a wrong pilot signal be given to the opposite line as in the case of Pilot Check Valves, but the quick draining of the Pilot port as well as the Valve port makes it easy for the poppet to sit quickly and give a leak free performance.Also in the event of a pressure spike caused by an external load or temperature increase, the relieved oil needs to be given a direct pathway to tank.system. Connecting A and B to T serve two purposes; the first is any spike at the outlet due to excess oil being deposited is prevented since it connects to tank (unlike in blocked centre where the oil would have nowhere to go). The second is that the inlet has a direct line to tank in the case oil is required in the prevention of cavitation which may permanently damage the pump.

Open Centre Spool: 

Open Centre spools have all ports open to each other in the central position. However, due to the smaller land width that needs to be maintained in order to allow flow, in the actuated positions, cross port leakages are highly probable.

One of the advantage of this spool is the pump directly unloads to tank. A circuit normally using a fixed-volume pump is used in conjunction with this type of valve. Ideally, the open center allows all of the pump’s flow return to tank with little or no back-pressure. This saves energy and reduces heat to the point that a heat exchanger is not necessary on most circuits. But the advantages end there.

 These spools find their use in applications where the valves are not taken up to the maximum flow and the cylinder is horizontal. Since the pump is constantly unloads to tank in the central position, clogging of the filter and restrictions in the line are possible which may lead to higher back pressures. Using this spool for vertical cylinder is impossible without holding valves as the ports are connected to tank and would bring the cylinder down. Even the use of Pilot Operated Check Valves or Overcentre Valves is debatable as the Pilot port may end up being pressurized due to restrictions in the tank line. 

Furthermore, even in horizontal cylinders, the restrictions in the tank line would put pressure on both the sides of cylinder (i.e. the head end as well as the rod end). Ultimately, in the neutral position the cylinder would gradually extend; a funny but true situation! This is because, the head end, having a greater area, would create a greater force forward for the same pressure as would the rod end. With this force imbalance, the cylinder would end up extending even in the central position!

These spools are also extensively used in vehicles where any external load, such as a pull or a push, can be adjusted without any cavitation. Equipment such as front-end loaders when they need to crawl without load or road paving machines where asphalt has to be laid in an even fashion find use of Open Centre Spools. In Front End Loaders or Side Dump Loaders needing to push material on the ground, the cylinder has to be biased to extend without there being any real pressure from the pump. At the same time, an obstacle such as a bump on the road causing the implement to get stuck may cause severe damage to the system. With Open Centre Spools, the cylinder is allowed to follow the contour of the path without being too rigid with minimal effort on the part of the pump thus reducing the power needed as well as protecting the implement and cylinders. 

Tandem Spool:

Tandem Spools connect the P port to the T port in the neutral position while blocking the A and B ports. Flow is directed from the P to the T port, ideally at a low pressure. Since there is no Use of the pump in the neutral position, two or more Tandem Centre valves can be connected in series to be sequentially operated. This may incite a lot of designers to opt for series connected Tandem Centre Valves, but beware! A circuit may look good on paper, but can run hot because of wasted energy. The spool is hollow, and ports P and T have cross drilled holes. This substantially increases the pressure drop of the valve for oil flowing from P to T. Because of the higher pressure drop most manufacturers’ catalogs show a lower nominal flow or higher pressure drop curve for tandem-center valves. Connecting these valves in series will only amplify the pressure drop since it is additive. Two or three valves connected in series could have pressure drops as high as 20-30 bar for large flows!

Tandem Centre Spools can beconnected in series to actuate systems one after the other using the same pump. The back pressure at T will be the cumulative back pressures of Valves 1, 2, 3 and 4

Another cause for concern is when the second spool is in operation, the load on the second P port is felt on the tank line of the first T port. In many Directional Control Valves, the tank line pressure is limited to 140 or 210 bar while the load on the P, A and B ports could be as high as 350 bar. This may cause damage to the valve if it is not used in a sensibly designed circuit.

Pressure Spool:

Pressure Spools have the P line connected to A and B to allow the actuator ports to remain pressurized in the neutral position. Equal pressures on both the actuator ports would not affect rotary actuators such as hydraulic motors. Liner actuators, however, would extend in the neutral
position due to unequal forces unless double rod cylinders are used. 

Although not very commonly seen in regular practice, they are used in machine tools with dual clamping systems where single acting cylinders are used. Here either one of the clamps has to be engaged with the work piece when it is being fed into the machine. While the operation is being performed, both clamps need to be engaged with the work piece for better stability.

Regenerative Spool:

Regenerative Spools have the P line connected to T in the center or neutral position to allow the pump to unload into the tank. In one extreme position, the A and B ports are connected to the P Line. This allows the flow coming out of the rod end of the cylinder (the ‘B’ line in this case) to feed oil into the cap end of the cylinder. This type of a system is quote common in hydraulics and is referred to as a “Regenerative Circuit”. 

(Left) Regeneration using two check valves. (Middle) Regeneration using a sequence valve and a check valve 

(Right) Regeneration using regenerative directional control valves.

Regenerative Circuits are used to save on the cost of the pump since oil is reused and a smaller capacity pump can be used for the same circuit (In cases of cylinders with 1:2 area ratios, the pump capacity will be slighter greater than half the original size required). Although these spools are slightly more expensive than the other spools mentioned earlier, there are huge savings on external valves to create the same regenerative circuit.

In the case shown in the spool allows the actuator ports to connect in the retract position. Cylinders would extend quicker than in other valves in this position due to outgoing flow being put back into the cap side. Although not very commonly seen in regular practice due to unawareness, they are used in many applications where the force of the cylinder is not in question but simply the speed. This is because pressurizing both ends of the cylinder would  mean a resultant extension, albeit reduced, force due to the pressure on the rod end acting against the pressure on the cap end. As in all cases, the regenerative spool also has its pro’s and cons!

For a view at our full range of Directional Control Valves, click here.

Click Here for Directional Control Valves - Part 1 
Click Here for Directional Control Valves - Part 3

Directional Control Valves - Part 1

Hydraulic power has found uses in a plethora of applications ranging from Industrial installations to mobile equipment. The increase in efficiency of the field over the last few decades has only endeared it more to system designers. The main use for hydraulics is to provide powerful motion to systems by amplifying the input in terms of power. Since the basic need for hydraulics is motion, it is but obvious that one of its most basic components would be one to control such motion. Directional Control Valves are used to start, stop and change the direction of fluid flow which results in the control of the end actuator providing motion.

Classifications 

As any hydraulic valve, Directional Control Valves have their classifications based on a number of factors. Some of them are listed below.

Internal Construction: Directional Control Valves can have varying internal constructions to allow and restrict flow paths from various inlets to various outlets. Some of the more popular ones are poppets/pistons, balls, rotary spools and sliding spools. Poppet and ball type constructions invariably have a seat within the valve to form a metal-on-metal contact and may be used to hold loads in place. They have near zero leakage from one port to another. Spool valves (either rotary or sliding) have certain amount of permissible leakages due to limitations in machining. They shouldn’t be used to hold loads in place. None-the-less they are still quite popular!

Flow Paths: The internal construction of valves provides various methods to allow and restrict flow paths. Hence the number of flow paths being governed also contributes to the valve’s classification. Valves may be two-way, three-way and four-way. Although the list is only limited by the design, valves with flow paths more than 4 are rarely used. Here the term “way” stands for a path.

Number of Ports: This is the cumulative count of all the entry and the exit passages for oil that the valve has. This number could be a humble two in the case of a simple poppet valve or more than six in the case of mobile valves or larger manifold mounted valves (CETOP8, NG22). The most common configuration is a four port valve with the pressure port, tank port and two actuator ports forming the entry and exit points. Additional flow ports are needed when there are external pilot and external drain ports or if the system has a “carry-over” configuration.

Actuation Methods: Flow paths can be selected by either external or internal actuation. Internally actuated Directional Control Valves are limited to check valves. External actuations can be by manual means (levers, buttons or foot pedals), mechanical actuators (such as cams, rollers, plungers/tracers or springs), electrical methods (either solenoids or electrical motors that get their signals from limit switches, push buttons or PLC controls) or by the application or release or hydraulic or pneumatic pressure.

Mounting of the Valve: Directional Control Valves can also be specified depending on the mounting patterns of the valve which may be flange mounted, piped thread mounted, straight thread mounted, cavity mounted or manifold interface mounted.

Size of the Valve: Directional Control Valves can also be specified depending on the size of the mounting pattern. Valve sizes can be defined by their flow capacities (given in lpm or gpm), their port or flange sizes (BSP, SAE etc.), their mounting plate size which are usually interchangeable (standard interfaces of CETOP such as 3, 5, 7, 8 or 10; NG sizes such as 03 mini, 03, 04 mini, 04, 06, 10, 16 or 22), the cavity size (SAE 08, 10, 12, 16 or 20) or manufacturer specific cavities with 2, 3 or 4 ports.

Construction 

Strictly speaking Check Valves and Pilot Operated Check valves are also a part of directional control valves but for the purpose of this article, we will only consider sliding-spool, subplate-type Direction Control Valves with more than 2 positions (ways).

Most Directional Control Valves are made up of three major parts or sub-assemblies:

Valve Body: The main valve body is made of non- porous cored cast iron or steel body with internal pathways connecting the various grooves to their respective external ports. The bore where the spool slides into place is usually ground and honed/lapped. The valve body does not generally differ by much in a particular size of valve regardless of the actuation or flow paths. The body may have various features depending on the specifications such as external gauge ports, interfaces for smaller valves in hydraulically actuated valves, speed control orifices for controlling the spool switching, orificing for draining moisture in pneumatic actuated valves etc.

Spool Assembly: The spool assembly, which consists of the spool, centering springs, washers and O-rings. Spools are made of case hardened steel and are mechanically or electrically moved. The movement of the spool restricts or permits the flow, thus it controls the fluid flow. Spools come in numerous versions depending upon the configuration of flow paths and the number of positions in the valve. Each spool has its own unique features and limitations which will be dealt with in a separate section later.

Actuation Assembly: The assembly for the actuation depends on the actuation method used. There are various levels of complexity from simply push button type valves to proportional electro-hydraulic vales with on-board controllers

Manual actuation valves usually have a simple lever assembly with either spring centering or detents to hold the position in place.

Mechanical actuation is a lot simpler in terms of assembly where the spool position is changed when the mechanical device is pushed inwards due to its actuation which in turn pushes the spool. The valve is reset by a spring on the opposite side.

Electrical Actuation is usually affected by solenoids although some applications call for servo motors as well. The most basic form of solenoids is of the “on- off’ type. This is because there is no mid-way control of the solenoids. They are completely off without a signal and on receiving the electrical signal, they are fully on. The version of solenoid valves that allow gradual variation in current are called proportional valves and will be dealt with in a separate article. The electrical actuation assembly for “on-off” type solenoid valves generally includes an armature of varying designs also colloquially called the “iron core” and an AC or DC solenoid coil.

Pneumatic Pressure actuation methods use air pressure to switch spool positions. The signals are received from external pneumatic systems operating simultaneously with the hydraulic system. Valves of pneumatic actuation need their body material to be of a non-corrosive nature so as to prevent rusting by the moisture content in the air. Internal assemblies of the valve are quite critical and rusting may cause jamming or sticking of the spool inside the bore. . In certain applications, due to the requirements of explosion resistance such as mines or oil rigs, the main spool is moved pneumatically. The solenoid valve, operated electrically, is kept safe in a place far from the explosive environment.

Hydraulic Pressure actuation is usually seen in the larger directional control valves where the power of the coil may not be sufficient to switch the spool due to large flow forces. In the case of mobile valves with bigger flows, a remotely operated joy stick is used. Joysticks are essentially one, two or four pressure reducing valves controlling different directions. At reduced pressure, the spring shifts the spool. Upon actuation, the spool shifts proportionally to the movement of the joystick. Hydraulic pressure actuation can also be obtained by using “double- decker” valves which are called so owing to the fact that a smaller directional control valve is mounted on top of the larger one. Pilot flow usually has to have a minimum pressure of 5 bar (70 psi) to switch the spool positions.

More on Spools 

The main advantage using a spools over a poppet in Directional Control Valve is that spool movements are immune to pressures within the valve. When a port is pressurised, the pressure acts in equal and opposite directions on the lands, this nullifies the overall effect of the pressure. Hence they can be shifted with a constant force by manual, mechanical, electrical, pneumatic or hydraulic means regardless of the operating pressure of the valve. Poppet Valves on the other hand face pressure imbalances due to pressure on one side and only light springs on the other. Hence premature movement of the poppet is possible when the port is pressurised.

Typically the spool is closely ground and matched with the valve body and is made from hardened steel, hardened to around 60 HRC or chrome plated and ground steel. The need for low leakages across the spool over a long service life necessitates the requirement for minimal diametrical clearances (ideally 5-10 μm) while geometrical tolerances of circularity, cylindericity and concentricity are to be exceptionally fine (2 μm). The spool has lands which block the oil passages and circular recesses which permit the flow. 

The lands have oil grooves which keep the spool “floating”. Without a hydrostatic oil film, the spool at rest will touch the sleeve causing abrasion, erosion and the creation of debris. If the pressure were to suddenly increase, the spool will be pressed against the bare surface of the sleeve causing metal on metal contact. With the oil grooves, there is a uniform film of oil maintained around the spool with transmits pressure equally around the circumference of the spool causing it to, in effect, float (Figure 5.b). The oil groves are generally square or ‘V’ shaped and about 0.5 - 0.8 mm deep. Square grooves with sharp corners prevent dirt particles from getting stuck in between the spool and the bore.

The number of oil groves per land is completely the designer’s choice. Higher number of groves gives a smoother movement but offers a higher leakage rate across the land which may not be appreciated in certain cases. Land length is also an important feature. Shorter the length of the land, higher is the chance of excessive leakage.

Spool positioning:

Spools that have not been shifted by the actuator(s) have to shift back into their original or dead positions. This is usually done using springs (except in the case of detented valves where the position is obtained by using the actuator).

Detented Valves: Some Directional Control Valves with manual actuation hold the valve in a particular position using a detent mechanism. These valves have notches on their spools. Using spring loaded pins the spool can be held in place by pushing the pin into a notch. Upon releasing the actuation mechanism, the spool does not shift back until the actuator is made to change the position. Shifting in detented valves is slightly jerky since the pin has to be forced out of the notch to change positions.

Spring-centered: In Directional Control Valves with two actuators on opposite sides, spring centered valves are used. These valves have springs on either sides. When one actuator is activated, the spring on the opposite end gains potential energy by being compressed. When the actuator is released, the spring expands, shifting the spool back. Springs on both sides balance each other out to adjust the spool to its centre position.

Spring-offset: Spring-offset in Directional Control Valves is seen in two position valves. There is only one spring present on the opposite side of the actuator that pushes it to the extreme position when the actuator is not active. There may or may not be a centre position in the spool, but it only comes into effect for a brief moment when the spool moves through it.

For a view at our full range of Directional Control Valves, click here.

Click Here for Directional Control Valves - Part 2
Click Here for Directional Control Valves - Part 3

Explosion Protection

Gases, vapours, mists and dusts can all form explosive atmospheres with air. Hazardous area classifications are used to identify places where, because of the potential for an explosive atmosphere, special precautions over sources of ignition are needed to prevent fires and explosions. 

There are various international bodies and directives that govern the classification of and precaution methods used in various explosion sensitive areas, some of which are -
● Appareils destinés à être utilisés en Atmosphères Explosivesis (ATEX, EU)
● National Electric Code (NEC, USA)

● International Electrotechnical Commission (IEC) 

● Dangerous Substances and Explosive Atmospheres Regulations (DESAR, UK) 

Hazardous area classification should be carried out as an integral part of the risk assessment to identify places (or areas) where controls over ignition sources are needed (hazardous places) and also those places where they are not (non hazardous places). Hazardous places are further classified in Zones which distinguish between places that have a high chance of an explosive atmosphere occurring and those places where an explosive atmosphere may only occur occasionally or in abnormal circumstances. The definitions of the Zones also recognise that the chance of a fire or explosion depends on the likelihood of an explosive atmosphere occurring at the same time as an ignition source becomes active.

For example, if a dangerous substance is being carried through a seamless pipe, and that pipe has been properly installed and maintained, it is extremely unlikely that the substance will be released. An explosive atmosphere would not be expected to occur from this source and the area surrounding the pipe would be non-hazardous. 

A spillage from a small bottle of solvent would release so little flammable material that no special precautions are needed other than the general control of ignition sources (for example, no smoking) and cleaning and disposing of the spillage. It would not be classified as a hazardous area. When considering whether hazardous area classification is necessary for “small” quantities of dangerous substances the actual circumstances of use and any specific industry guidance should also be taken into account.

Dangerous substances in small pre-packaged containers for sale, display, etc. in retail premises would not normally require the area to be classified as hazardous. However we would expect a hazardous area classification to be carried out for prepackaged containers held in large quantities e.g. in warehouses.

Classifying Hazardous Areas into Zones

Once an area has been identified as hazardous it should be classified into zones based on the frequency and persistence of the potentially explosive atmosphere. This then determines the controls needed on potential sources of ignition that may be present or occur in that area. These controls apply particularly to the selection of fixed equipment that can create an ignition risk such as solenoid valves; but the same principles may be extended to control the use of mobile equipment and other sources of ignition that may be introduced into the area (for example, matches and lighters) and the risks from electrostatic discharges.




The directives define a place where an explosive atmosphere may occur in quantities that require special precautions to protect the health and safety of workers as hazardous. Classifications are dependent on the exposure of the area to explosive atmosphere or ignition points and may be continuous exposure, occasional exposure or unlikely to have exposure. A place where an explosive atmosphere is not expected to occur in quantities that require such special precautions is deemed to be non-hazardous. 

The term “not expected to occur in such quantities” means that employers should consider the likelihood of releases of explosive atmospheres as well as the potential quantity of such releases when considering area classification. So if a release is extremely unlikely to occur and/or if the quantities released are small, it may not be necessary to classify the area as hazardous. Site-specific factors should always be taken into account for such cases.

Gases, vapours and mists

For gases, vapours and mists the zone classifications are: 

Zone 0: A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.

Zone 1: A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.

Zone 2: A place in which an explosive atmosphere consisting of a mixture with air of dangerous  substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Dusts

For dusts the zone classifications are:

Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.

Zone 21: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Equipment Categories for Hazardous Areas

Special precautions need to be taken in hazardous areas to prevent equipment from being a source of ignition. In situations where an explosive atmosphere has a high likelihood of occurring, reliance is placed on using equipment with a low probability of creating a source of ignition. Where the likelihood of an explosive atmosphere occurring is reduced, equipment constructed to a less rigorous standard may be used. Equipment is categorised (1, 2 or 3) depending on the level of zone where it is intended to be used.

Equipment categories and zones
The hazardous area zone classification and corresponding equipment categories are:

Zone 0 or Zone 20 - category 1 equipment

Zone 1 or Zone 21 - category 2 equipment
Zone 2 or Zone 22 - category 3 equipment

Method of Explosion Protection

Designed to prevent any means of ignition arising

 - Increased Safety Protection (e)

In this method any components that produce sparks as part of their normal operation are excluded from the equipment. Components are designed to substantially reduce the likelihood of fault conditions that cause ignition. This is done by reducing and controlling working temperatures, ensuring the electrical connections are reliable, increasing insulation effectiveness, and reducing the probability of contamination by dirt and moisture ingress.

 - Non Sparking Protection (nA)

Precautions are taken with connections and wiring to increase reliability, though not to as high a degree as for ‘e’. Where internal surfaces are hotter than the desired T rating discussed later, they can be tightly enclosed to prevent the ready access of a flammable atmosphere into the internal parts. This is the "restricted breathing enclosure" method. The use of this method also means that high ingress protection ratings of IP65 and above are built into the design.

Designed to limit the ignition energy of the circuit

 - Intrinsic Safety Protection (ia/ib)

In this method the circuit parameters are reliably controlled to reduce potential spark energy to below that which will ignite the ambient gas-air mixture. The coding ‘ib’ and ‘ia’ denotes that the unit will not cause ignition of explosive atmosphere under normal operation and with one and two faults present in the circuitry respectively. This method does not protect entirely against the local over-heating of damaged connections or conductors and these should be kept sound and suitably protected against damage.

Designed to prevent the flammable mixture reaching a means of ignition

 - Oil immersion protection (o)

This is an old method mainly used with switchgear. Any spark generated by the operation of the switchgear is formed under oil and the venting is controlled.

 - Pressurised protection (p)

There are two protection methods. One maintains a positive static pressure inside the equipment. The other involves a continuous flow of air or inert gas to neutralise or carry away any flammable mixture entering or being formed within the enclosure. Essential to both methods are monitoring systems and purging schedules. 

 -  Encapsulation Protection (m)

In this method the potentially incendive components are encapsulated so that the flammable atmosphere is excluded. The method also involves the control of the surface temperature under normal and fault conditions.

Designed to prevent any ignition from spreading

 - Flameproof Enclosure Protection (d)

In this method the potential incendive components are contained within an enclosure. Although the flammable atmosphere can enter the enclosure, any resulting explosion is contained and its transmission outside the enclosure prevented.

 - Powder Filling Protection (q)

This method involves the enclosure to be filled with an inert powder. The enclosure is also vented. The method is primarily of use where the incendive action is the abnormal release of electrical energy by the rupture of fuses or failure of components such as capacitors. 

Explosion Groups

Explosive gases, vapors and dusts have chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.

Apparatus marked IIB can also be used for IIA gases. IIIC marked equipment can be used for both IIIA and IIIB. If a piece of equipment has just II and no A, B, or C after then it is suitable for any gas group.

Temperatures
Additional information relating to the process that involve the dangerous substances should also be taken into account, including the temperatures used in the process, as this will influence the nature and extent of any release, and the extent of any subsequent hazardous areas. Some substances do not form explosive atmospheres unless they are heated, and some liquids if released under pressure will form a fine mist that can explode even if there is insufficient vapour. 

Product Selection and Marking

Using the above information, products can be selected as per the requirement of the zone. An example of Electrical Equipment designation is given below for ATEX and IEC methods of classification.

Risk Assessment

Identifying hazardous or non-hazardous areas should be carried out in a systematic way. Risk assessment should be used to determine if hazardous areas exist and to then assign zones to those areas. The assessment should consider such matters as:

1. the hazardous properties of the dangerous substances involved;
2. the amount of dangerous substances involved;
3. the work processes, and their interactions, including any cleaning, repair or maintenance activities that will be carried out;
4. the temperatures and pressures at which the dangerous substances will be handled;
5. the containment system and controls provided to prevent liquids, gases, vapours or dusts escaping into the general atmosphere of the workplace;
6. any explosive atmosphere formed within an enclosed plant or storage vessel; and,
7. any measures provided to ensure that any explosive atmosphere does not persist for an extended time, e.g. ventilation.

Taken together these factors are the starting point for hazardous area classification, and should allow for the identification of any zoned areas.