Pressure Change Due to Temperature in Closed Fuel Piping System

[the1falconer]

I have a question regarding fuel terminal transfer manifold design. I came across an old design that had two valves directly connected to each other. The first valve is a check valve and the second is a general twin seal plug valve. In order to transfer product from one tank to another, the plug valve would be opened to allow product from the tank discharge branch line into the manifold. Opening a second plug valve on a separate branch line would then send the product to the inlet of the desired tank. The product entering the manifold first flowed through the check valve and then through the plug valve. As I mentioned previously, both valves were connected directly to each other with a flanged connection. The back check would prevent product from entering the discharge line from the tank. The plug valves are typically closed which leaves a small area between the seal of the plug valve and the closed flap of the check valve where product can get trapped. Is there a reasonable way to calculate the pressure difference resulting from temperature changes throughout the day?

The piping is 3" Sch. 40 carbon steel, and the valves and flanges are 150# Class. Based on a similar terminal, I am assuming a maximum operating pressure of 75 psi can be achieved and used for the initial pressure trapped between the two valves. A temperature change of 40°F - 50°F seems reasonable for a bounding temperature change throughout the day.

Based on my limited research in similar forums, I have seen a rule of thumb of 100 psi pressure increase for every 1°F increase. This would put the pressure well over the design limits of the piping components (valves, gaskets, etc.). The old design I came across was in use for approximately 60 years. How did it not have catastrophic failures at these connections? Does anyone on this forum use a similar setup. A relief was not used between the two valves. I would appreciate any insight on this specific configuration. I think the best solution would be to switch the order and have the plug valve first followed by the check valve or eliminate the check valve altogether since the plug valve is typically closed unless transferring from one tank to another. Thank you.

 

[LittleInch]

This basically relied on the check valve not fully sealing.

Depending on the type check valves often need a little bit of flow or differential pressure to start to seal. Even then it's very rare for a check valve to have good seals or the ability to get a high class shut off.

What will happen in this case is that when flow stops there is a small gap in the check valve sealing. This allows the very very small actual volume increase to seep past the valve without causing it to seal.

However if you've got longer distances between closed valve and check valve or if the check valve is pressurised before the isolation valve closes and it's a new valve then you can get pressure build up.

It would be better practice to allow the check valve to self relieve and drain and this normally means putting it on the downstream side of the isolation valve and upstream from the drain valve / vent valve / pressure or thermal relief.

A check valve is often good practice t prevent excessive back flow if the valve operation is mis operated or in the period between valve open to valve close. Those general twin seals are good valves.

But if its worked for 60 years....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[the1falconer]

Thank you very much for the reply LittleInch. This is helpful.

 

[georgeverghese]

Suggest installing a single thermal RV on the manifold. All check valves leak slightly, so no need to move the check valve downstream of the feeder line plug valve.

 

[the1falconer]

Thank you George. There was a relief valve assembly both upstream and downstream of the valve pair.

 

[lilliput1]

Isn't the manifold supposed to be drained after each transfer so the next batch will not be contaminated?

 

[the1falconer]

The manifold is approximately a 10'-0" long segment of 3" sch. 40 pipe. Each of the isolation valves on the branch lines that connect to the manifold are typically in the closed position (unless transferring product from one tank to another containing the same product). Since the manifold is used to transfer gasoline and diesel products, there is a chance for some cross contamination from residual product left in the manifold. However, the storage tanks that are used at the terminal each have a capacity greater than 1,000,000 gallons. Therefore, the small amount of gas that could get into a diesel tank or vice-versa will have a negligible impact on the product in the storage tanks.