10 replies to this topic

[Engineer87]

Hi respected members!

 

I have a little problem here. When I am calculating back flow through a check valve I typically use 100% of the check valve flow capacity for relief / psv sizing. However, I heard from some other process engineers that it could be much less. Somewhere between 10-20% of the check valve flow capacity is used for relief / PSV sizing.

Does API 520/521 isays anything about that?

[christopher.choa]

API 521 5th Ed Sec 4.3.4.2 states that "When sizing a pressure-relief device to prevent exceeding the allowable accumulation of the protected  equipment for the latent check-valve failure, the reverse flow rate through a single check valve may be determined using the normal flow characteristics (i.e., forward-flow Cv) of the check valve. If the check valve Cv is unavailable, one may conservatively assume that the check valve is not there by taking no credit for its flow resistance."

 

10% of the cross sectional area that you are referring is for double check valve failure.

[aroon]

Hi Christopher,

 

Is your above statement is valid even if check valve is designated as safety critical?

[jrtailor09]

Dear Engineer87

 

As per API standard 521 clause no. 4.3.4.4 Pressure consideration for the series of back flow prevention

“Two back flow devices in series are sufficient to eliminate significant reverse flow”. There is no specific experience or company guidelines exist , we have estimated the reverse flow through series check valves as the flow through a single orifice with a diameter of 1/10^th of the largest check valve nominal flow diameter.

 

Regards,

Jatin

[Engineer87]

Thanks Christopher and Jrtailor for such helpful info

[Bobby Strain]

For two check valves, they must be a different kind from eash other. And note that 1/10 of the diameter, which is 1% of the area!

 

Bobby

[Pilesar]

A few comments:

1) Any expectation that a check valve will prevent back flow depends on it being regularly inspected and maintained. Inspection and maintenance is often never done until after the check valve fails. That is too late if you are allowing credit to install a smaller relief valve.

2) API 520 and API 521 are guidelines. Safe design of your relief system is NOT the responsibility of API. Sound engineering judgment is required.

3) I hope anyone sizing a PSV will at least read and understand the references in context and not rely solely on excerpts of one or two sentences from an API standard presented in an online forum. Designing safety systems is one of the most important engineering functions because every process we design can eventually fail.

[fallah]

Engineer87,

 

In practice, to calculate the reverse flowrate and overpressure due to back flow in check valve failure case, a single check valve will be considered to be in full open position.

 

In two or more check valves in series, it would always be assumed that one check valve is in full open position and the other will leak. Reverse leakage rate will normally be 10% of the forward flow.

[aroon]

Hi Fallah / all,

 

Is this first statement valid even if check valve is designated as a safety critical (mandatory periodic maintenance/check provision) by operation?

[fallah]

aroon,

 

If it is well documented and reflected in relevant operating manual and also be considered as a safety critical item in operating and maintenance schedule, one might take a credit to reduce the reverse flow in single check valve failure case from what mentioned in the first statement...

[aroon]

Thanks Fallah.