Volume Tank for Control Valve

Hi there,

This not an easy thing to answer. You need to know how critical the service of the valve is. You would usually discuss this in a Hazop and from there determine whether you need a volume tank/air receiver for the valve. You would also then decide on the failure position of the valve.

In my experience I have only used volume tanks on ceartain critical valves that would for example open to a flai header. I have use the volume tanks where, because of the header size, I had to stagger the valve reaction. i.e. valve one to open first then after a while valve two and so on.

You need to know the air consumption of the valve and all it's components to size the volume tank. You should be able to get this data from the valve manufactuer. I would then also add 25% to that so that I would have a good safety margin.

Regards

Craig

Dear Josej

1) You must have a Volume Tank when using piston type actuators without spring in critical applications. Example: anti surge control.

There are basically two types of actuators used in control valves: spring & diaphragm and piston. In case of shortage of air supply, spring will drive the valve into a safe position due to spring action: close or open, depending on the process safety.

Some piston actuators do have spring return action. Some do not have and require use of air supplied by volume tanks.

2) The amount of air supply to stoke 03 times depend on size and type of actuator. This information have to be requested from actuator manufacturer. Recommend purchase of control valve and volume tank from same manufacturer.

1. Reduce your operating pressure and cycle the system to find the lowest pressure that gives a satisfactory cycle. Note this pressure, Pc, and reset the regulator to normal pressure.
2. Get a tank that is suitable for your operating pressure. Grainger sells inexpensive 5 gallon air tanks rated for 125 psig for around $50.
3. Plumb a suitable check valve, the tank and a gauge into your supply before the regulator so that the tank pressure is the highest available and cannot exhaust when supply is exhausted or disconnected as in an air failure.
4. Charge the system including the tank and note the tank gauge pressure = P1.
5. Cycle the system without external supply 3 times and note the residual tank gauge pressure = P2. If P2 is above critical you are good.
6. The easy way is to cycle your system three times and see if the pressure drops below critical pressure. If so add a tank or use a larger one and repeat steps 1 through 5.
7. If you want to calculate an adequate but minimal reserve tank calculate tank volume that you are using. Vo, Gallons/7.48 = Vo Cu Ft.
8. For an example allow me to use 5 gal / 7.48 = .67 Cu Ft = Vo and for P1 (100 + 14.7) = 114.7, P2 = 53 + 14.7 = 67.7, Pc = 40 + 14.7 =54.7
9. Vo x Pc / P2 = Vr then .67 x 54.7 / 67.7 = .54 cu ft (4 gallons) would be the minimum volume required.
10. Add a safety factor to the critical low pressure for temperature, margin of error and the system volume outside the tank.

Be Well, Tom