IMO, each Company has it's own guideline for the application of double checks piping arrangement. For example, the Municipal Utility piping branch to Refinery, the Utility piping system connected temporarily to the process piping, or required by HAZOP review for the process line, etc.
In petro-chem and refinery facilities I only see double check valves used when there's a need to reduce the magnitude of an overpressure scenario that is caused by reverse flow ("check valve failure"). Per API 521, one shouldn't rely on a single check valve to render a reverse-flow scenario non-credible. But, if two check valves are installed (and they are periodically inspected/tested) then one may greatly reduce the expected flow rate during a check-valve failure scenario. One may calculate the reverse flow by regarding the double check-valves as a single orifice that has a bore diameter that is 10% of the diameter of the check valves.
A check valve allows flow in one direction and automatically prevents backflow when fluid in the line reverses direction. We typically use the dual check valve arrangement when a fluid is sent from a lower pressure system to a higher-pressure system to prevent reverse flow. The design rating of the equipment downstream of the check valve is the one considered when sizing the check valve and the associated relief valve (not fully sized) connected on the upstream equipment of the check valve.
Think of this: if only one check valve is installed, termination of flow accompanied by failure of that check valve in the discharge line will result in a reverse flow. When the fluid has been displaced into a suction system, and a high-pressure fluid enters the upstream equipment, serious overpressure may occur. A single check valve is usually considered acceptable unless there is a potential for the backflow of high-pressure fluid to create pressures that exceed the test pressure of the upstream equipment. That is why there should be a double check valve arrangement for potential reverse flow overpressure scenarios.
Note that two back-flow-prevention devices in series, when inspected and maintained, are sufficient to eliminate significant reverse flow. But since industrial guidelines suggest an assumption for 10% backflow in a well-maintained dual check valve safeguard, it is recommended to pair this with a relief valve (not fully-sized) installed in the upstream equipment.
In any case, you may also contact us (contact@saltegra.com) for a FREE consultation. We'll be glad to assist you.
I hope this helps!
Reference: API Standard 521 - Guide for Pressure Relieving and De-Pressuring Systems
The difference between a double check valve and a back-flow preventer valve is that in a back-flow preventer the space between the two check valves is kept at a lower pressure than the inlet inlet to the first check valve. This is done with a regulator valve that will drain to a lower pressure. Back-flow preventers are very commonly used to connect potable water lines to process water lines and those generally drain to ground. Two check valves do not a back-flow preventer make!
I agree with the response of @mdmendoza and @don1980 regarding the overpressure scenario. And also 10% value is almost universally agreed. But how this 10% need to be applied is sometimes not very clear. @mdmendoza mentions that it is 10% backflow and @don1980 mentions that flow to be calculated based on 10% diameter. Please note that 10% diameter is 1% area. Any views?
Also need to mention here that most companies ensure that type of check valves used in such cases are dissimilar to each other. This is to ensure similar events will not cause the failure of both check valve at the same time.
The referenced API 521 paragraph gives the user two options for quantifying the reverse flow when two check valves are used. The flow can be calculated as I mentioned above, or one may use 10% of the normal forward flow.
Razookm is correct that many companies have internal requirements stating that the two check valves must be dissimilar. That's a good recommendation but it's not an API 521 requirement.
This discussion / argument appears in every HAZOP i've ever been in and has about as many conclusions.
For me it's all about reduction of risk / reduction of back flow.
I've seen some chairman try to say that a check valve isn't accepted as a risk reduction measure which I think is going too far.
Whilst two check valves in series are commonly seen on outlets of potable water systems when connected to a downstream "dirty" system, the consequences of failure seem low.
Potentially for a refinery they are very serious.
But putting two in series for high consequence events ( severe over pressure of an upstream system) should be seen as a risk reduction measure, not a risk elimination measure.
The 10% I've seen is usually 10% of flow, but it has varied to be 10% of the area.
So when and why?
When - Where failure of a single check valve would either require a massive increase in relief capacity or where failure of the upstream equipment would be catastrophic. Having a large pressure difference between rated pressure and downstream pressure is also an indication of where to apply this.
Why - Failure rates, especially if different types are reduced.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
