5 replies to this topic

[kpavan]

Dear All,

I would like to have some views and comments regarding series of check valve failures as a credible overpressure scenario while evaluating a PSV's scenario.
I am trying to evaluate a check valve failure case which has a series of Wafer type check valves but installed on 4" and 6" lines.

As per API-521, clause 4.3.4.4
Where no specific experience or company guidelines exist, one may estimate the reverse flow through series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal flow diameter.

My query is, can i take 10% dia. of 6" line as orifice size (as check valves are dissimilar in terms of valve size) or should i consider for 100% dia (as check valves are of same Wafer type) and do calculations based on Line Hydraulics and find the relief flow?

I would appreciate all your suggestions.

Regards,
Pavan.

[fallah]

Dear All,

I would like to have some views and comments regarding series of check valve failures as a credible overpressure scenario while evaluating a PSV's scenario.
I am trying to evaluate a check valve failure case which has a series of Wafer type check valves but installed on 4" and 6" lines.

As per API-521, clause 4.3.4.4
Where no specific experience or company guidelines exist, one may estimate the reverse flow through series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal flow diameter.

My query is, can i take 10% dia. of 6" line as orifice size (as check valves are dissimilar in terms of valve size) or should i consider for 100% dia (as check valves are of same Wafer type) and do calculations based on Line Hydraulics and find the relief flow?

I would appreciate all your suggestions.

Regards,
Pavan.

In my opinion:

1-Estimate nominal flow diameter for each valve (obviously this diameter would be lower than relevant line size)
2-Take the largest value among estimated values in previous stage
3-Take 10% of the value obtained in stage 2 as orifice diameter
4-Calculate the reverse flow based on orifice diameter of stage 3

Regards

[kpavan]

Dear All,

I would like to have some views and comments regarding series of check valve failures as a credible overpressure scenario while evaluating a PSV's scenario.
I am trying to evaluate a check valve failure case which has a series of Wafer type check valves but installed on 4" and 6" lines.

As per API-521, clause 4.3.4.4
Where no specific experience or company guidelines exist, one may estimate the reverse flow through series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal flow diameter.

My query is, can i take 10% dia. of 6" line as orifice size (as check valves are dissimilar in terms of valve size) or should i consider for 100% dia (as check valves are of same Wafer type) and do calculations based on Line Hydraulics and find the relief flow?

I would appreciate all your suggestions.

Regards,
Pavan.

In my opinion:

1-Estimate nominal flow diameter for each valve (obviously this diameter would be lower than relevant line size)
2-Take the largest value among estimated values in previous stage
3-Take 10% of the value obtained in stage 2 as orifice diameter
4-Calculate the reverse flow based on orifice diameter of stage 3

Regards

Thanks Fallah for your quick reply,
I have done those calculations as described by you. But the doubt that is hunting me is if my first check valve fails which is a Wafer type, then how is the possibility of the same type of valve to fail (but installed in different size) which is in series to the previous one.

As per API 521, clause 4.3.4.4,
The quantity of back-flow leakage through check valves in series depends on the types of check valves, the fouling nature of the fluid and other system considerations.

So, is it apt to take 10% of the bigger check valve nominal diameter in such a case.

Thanks & Regards,
Pavan

[fallah]

So, is it apt to take 10% of the bigger check valve nominal diameter in such a case.

IMO,for the system you mentioned (including a series of Wafer type check valves),yes,even though would be a little bit conservative.

Regards

[Poosticks]

Thanks Fallah for your quick reply,
I have done those calculations as described by you. But the doubt that is hunting me is if my first check valve fails which is a Wafer type, then how is the possibility of the same type of valve to fail (but installed in different size) which is in series to the previous one.

If you have two check valves in series then the chances of them both failing together are more likely if they are both of the same type - there could be some system phenomenon that is causing an incompatibility with the wafer mechanisms which would cause simultaneous failure.

The best practice would be to install two different types of check valves.

[kpavan]

Dear Poosticks,

Thank for your reply.
I agree that the best practice is to install two different check valves. But the plant I was talking about commissioned 10 years ago. And at that time they have taken all the Wafer type of check vlaves. Even though both are of same type of valves, they are of different sizes (installed on 4" and 6" lines). Doesn't that make them dissimilar valves? (so that 10% of bigger valve diameter can be taken into consideration.)

Regards.