Decompression of Presses

Deciding between a loud system and a leaky one when decompression is involved need not be as hard as you think.

 A lot of the time, we get complaints from press manufacturers that come to us with the following problem:

"I have a cylinder on my press and every time I have to unload it, there is a loud banging sound."

If we consider that all the other mechanical parts of the press are working fine (i.e. the problem lies with the hydraulics), it is most likely caused by the uncontrolled decompression of the hydraulic fluid. The problem arises because hydraulic fluid is not perfectly rigid.

One of the popular misconceptions with hydraulics today is that fluids are incompressible within systems. The fact is that they are compressible, albeit the compressibility is not as large as in the case of gasses. The ratio of a fluid's decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity. The bulk modulus for most hydraulic fluids is approximately 17,000 kg/cm2 which results in a volume change of around 0.4% per 70 bar (1000 psi). When the change in volume exceeds 160 cc, decompression must be controlled.

The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work. If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path. Uncontrolled decompression stresses hydraulic hoses, pipes and fittings, creates noise and can cause pressure transients that damage hydraulic components. This is similar to a balloon burst by means of a pin when the compressed air is opened to atmosphere rather than allowing the air to be let out smoothly.

Figure 1:Successive loading of a cylinder leads
to a pressure rise in the fluid contained within.

Consider a system as shown in Figure 1 below. The cylinder bore is 100 cm² and the height is 10 cm.If we load the cylinder with a 7000 Kg weight the height which was 10 cm initially will now become 9.9 cm. Thus there is a volume reduction of 1% (or 10cc). Here an assumption is made that the system is not perfectly rigid i.e. cylinder diameter does change. Add another 7000 Kg and the height will reduce to 9.81 cm, and another 7000 Kg would make it approx 9.712 cm and so on. Thus increase in weight would reduce the volume.

Similarly if we started with 21000 kg of weight on a closed cylinder with 100cm sq area and 10 cm height, and we remove 7000 Kg weight, the height would increase to 10.1 cm. remove another 7000 Kg and the height would be about 10.2, remove another 7000 Kg and the height would be about 10.3 cm.

Note: The relationship between the pressure and the volume is not linear here but exponential. Adding 7000 bar of pressure will not compress the fluid to zero volume.

Figure 2: An enclosed fluid being pressurised will
develop potential energy similar to that of a compressed
spring. This energy is then released instantaneously as
noise and vibrations when the fluid is decompressed

Now we take another example shown in Figure 2 below. We have an enclosed chamber of volume 100 cc in which a pump is pumping oil. The Relief Valve is set at 350 bar and as soon as the pressure in the system reaches 350 bar, we would find the pump blowing over the Relief valve. If the pump is shut off, the Check valve will hold the pressure in the tank at 350 bar. In order to lower the pressure all that is needed to be done is to open the Shut Off valve. How much oil should come out of this valve? If we remove 1cc of oil, the pressure would drop to 280 bar; another removal of approx 1cc would bring it to 210 bar; another removal of approx 3cc will bring it to zero pressure. This phenomenon is what is used in decompression circuits.

The fluid needs to be removed gradually. If the shut off valve is opened suddenly, the fluid will be exposed to atmospheric pressure and approx 5cc of oil (the amount required to bring the oil to atmospheric pressure) will escape making a big noise and the chatter of system. 

There are two popular methods used to achieve decompression of a circuit.

1. Using a Pilot Operated Check valve with an inbuilt decompression feature (Figure 3).

2. Using a poppet type decompression valve for allowing few ccs of oil to escape followed by the main opening of the pilot check valve holding the load in place

Figure 3: Pilot Check Valves with in-built decompression feature.

Figure 4: Decompression using PO Check with
internal decompression feature

Use of a Pilot Operated Check valve with decompression feature has its advantages. It gives a compact system and a cheap one (Figure 4). However, the decompression feature in a Pilot Operated check valve increases the area ratio of the valve. A standard Pilot Check valve has an area ratio between 3 and 4 (5 in extreme cases). Thus a 300 bar system will open between 75-100 bar. Use of the decompression type Pilot Check valve increases the area ratio to about 20 (25 in some cases). Therefore a 300 bar system fitted with Decompression Pilot Check with area ratio of 25 will open at 12 bar and that with area ratio of 20 will open at 15 bar. This will reduce the loud bang in the system which occurs when an un-decompressed Pilot Check valve opens and the system will become smoother. This looks perfectly fine and is acceptable to customers.

Now take the same system when it is set to 100 bar (as in the case of many pressing applications). The same decompression type Pilot Check valve (with area ratio between 20 to 25) will open the valve between 4-5 bar. The standard pressure drop across a moderately clogged filter is 2 bar which an excessively clogged filter may issue pressure drops of even 4 bar. 

This is notwithstanding the various other pressure drops caused due to hydraulic fittings such as elbows, contaminants in the pipe and/or other unwanted resistances to flow in the tank line. Thus one tends to think that there is some problem with Pilot Check valve, ignoring the fact that the problem lies with the circuit design. These systems will work only when the system pressure is always 250 bar+. 

Figure 5: Isolated Holding and Decompressing circuits

The solution to the above problem is to isolate the Decompression system from the Pilot Check system using a solenoid valve as shown in Figure 5. Along with the solenoid valve, an orifice of size 0.8 to 1.6 mm is used as well. The potential energy of the compressed fluid is converted into heat by metering the compressed volume of fluid across the orifice. The solenoid valve is a poppet type valve with metal on metal contacts that ensure that the valve has zero leakages. The valve opens before the main stage DC valve and releases a few drops of oil. This ensures that the cylinder pressure is near zero when the main solenoid valve opens. This allows the Pilot Operated Check (with standard pilot ratios from 3 to 4) to operate without any noise or vibration problems. This system, albeit costly, works well through all pressure ranges; whether in a high pressure system of 300 bar or a low pressure one of 100 bar.

Tucson offers various types of decompression solenoids in either a line mounting or a subplate mounting. Tucson recommends a DC solenoid as this has a longer life over its AC counterpart. This is due to the rectifier plug embedded on the solenoid coil could get damaged if there is a voltage spike. The solenoids in Tucson are offered in 12V and 24V DC and 230V AC. Many major press manufacturing units have changed over from a decompression Pilot Check Valve to multi stage Solenoid Decompression valves, which are only a shade more expensive but reliable, accurate and long lasting.