At my workplace, we have been insta 1

[maceca]

At my workplace, we have been installing them in various locations, but it seems we have lost our way a bit.
We are putting doble check valves in drainage points to prevent backflow when the vessel pressure drops below the drainage system’s pressure.
I would like to know where exactly should doble check valves be exclusively used?

 

[LittleInch]

Closed drain?

Why is the drainage system pressure > Atm?

Seems rather OTT to me, but care to explin a bit more ( fluid, industry, P&ID, why, that sort of thing

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[georgeverghese]

Why is there high pressure in your drain vessel? In some of the better run larger oil companies, draining of process vessels follows clear cut operating guidelines.

Draining of vessels is only to be done AFTER depressurisation of vessel volatile inventory to the vent /flare system. If your operators dont follow these guidelines, then you have requests for double check valves (and / or restriction orifices) in drain lines etc to prevent backflow into LP vessels. Then you have blockage of the drain line caused by double check valve / RO.

Run some seminars for engineers and Operators to present safer depressurisation / draining procedures, so you avoid all these problems. Drain lines should be normally CLOSED with spade in CLOSED position in normal operation. Most oil/gas engineering contractor process design engineers will also be able to assist you with more details.

There is frequent misoperation of plant drain systems resulting in major incidents and these seem to continue. Operators and Managers tend to abuse and neglect closed drain systems for obvious reasons. A misoperated drain system will at some time bite back.

 

[mk3223]

Many incidents show that the double check valves may not be totally reliable to prevent the backflow in the process as it’s intended. And the additional safeguards may be required per PHA or HAZOP study.

 

[LittleInch]

Check valves in this service are liable to foul up and not seal due to the low back pressure.

Spring loaded ones don't work either as the DP is often very low and you want the vessel or pipe to drain.

Look at what George is saying.

You have lost your way and IMHO you should start from the beginning.

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Also: If you get a response it's polite to respond to it.

 

[maceca]

Sorry for a few details.
Normally these scenarios are in compression system where we drain second stage scrubber to first stage scrubber, for exemple. Second stage is 30 bar and first stage is 10 bar. Therefore, when we stop the system and the second stage depressurizes with the first stage pressurized. In the same cases, the first stage scubber is common to two or more compressors. We Normaly use a safety scrubber for multiple compressors. Them this firt stage continues pressurized.
it is possible occour reverse flow from first stage to second stage depressurized. in drain line of scrubber we have a LV and SDV valves, LV controlled by LIC and SDV closed by Low low level switch.
this drainage is continuous.

 

[maceca]

More details.
This fluid is condensed and, in some cases, oily water and/or oil is continuously drained from equipment with high pressure to equipment with low pressure, however, when the first is depressurized, the second becomes at a higher pressure and reverse flow can occur.

 

[maceca]

In other cases we are using doble check valves at stand by equipment to prevent back flow from discharge common line.
despite using SDV valve to isolate.

 

[LittleInch]

Cascading pressure on drain system is unusual. I assume there is some sort of pressure control in normal operation?

But why?

Looks like a recipe for disaster to me.

But things will improve massively with a P&ID or a decent sketch of what you have.

My issues still stand. Drain are by their very nature dirty and you simply won't seal or can get severely blocked.

Generally speaking reliance on NRVs is not reliable and shouldn't be used as part of normal operations and certainly not as any form of isolation when the pipe is depressurized and anything unbolted.

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Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Ok, so these are continuous process drains, not maintenance drains.

"it is possible occour reverse flow from first stage to second stage depressurized. in drain line of scrubber we have a LV and SDV valves, LV controlled by LIC and SDV closed by Low low level switch. this drainage is continuous."

Agreed, one way is to use double check valve to prevent reverse flow into the depressured stage 2 scrubber. Another way is to account for reverse flow in case of failure of single check valve and SDV at the PSV on stage 2 scrubber. Use 10% of the nominal cross sectional flow area of the check valve to model the reverse flow leak and treat this as a restriction orifice. If you have high high level at stage 2 scrubber, then shutdown and depressure the common stage 1 scrubber also. In some Companies, double check valve only is used for small chemical injection lines but not for main process systems.

"This fluid is condensed and, in some cases, oily water and/or oil is continuously drained from equipment with high pressure to equipment with low pressure, however, when the first is depressurized, the second becomes at a higher pressure and reverse flow can occur."

Same approach as previous can be used here also

 

[LittleInch]

In recent HAZOPS ~I've been in the yoften seem to disregard check valves as a mitigation against reverse flow.

The 10% rule is often used and then with double ones this goes down to 1%, but it's not regarded as "isolation" and all the safety systems, reliefs etc need to assume that even double non returns will leak to a certain extent.

Hence they should only be used to reduce the flowrate of a reverse flow which gives you time to either isolate the line using an isolation valve or limit the relief flow rate if this would end up over pressurising the component which is normally upstream. IMHO, they should not be used as a quasi isolation valve.

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