7 replies to this topic

[mido]

Can we use two dissimilar check valve as a protection from high pressure between 600# ansi system and 150#ansi system?

[latexman]

I'd want to see the P&ID before saying anything.

[Pilesar]

I don't know who 'we' is in your question. But if 'we' includes me, I say check valves will not define a pressure spec break. Check valves may be useful to limit reverse flow, but I would not consider them reliable as a pressure barrier.

[Bobby Strain]

The usual solution is to disconnect the two systems.

 

Bobby

[Chemitofreak]

Can we use two dissimilar check valve as a protection from high pressure between 600# ansi system and 150#ansi system?

 

For me one can isolate two systems only with the help of block valves.

 

Check valves or for that matter two dissimilar Class I check valves cannot be considered reliable for isolation.

[breizh]

Hi,

To me isolation is either block valves or disconnection .

Breizh

[Jiten_process]

@mido,

 

If I understand your scenario correctly, you want to protect your low pressure (150#) system from overpressurization due to reverse flow from high pressure system (600#) and you want to employ two dissimilar check valves for that matter. 

 

API and Shell allows credit of two dissimilar check valve in series to prevent "BULK REVERSE FLOW".  However, you still need to size your low pressure system relief valve for reverse relief flow which can pass through orifice area equivalant to 1% of check valve cross sectional area taking max. DP during relief situation i.e. PAHH on hig pressure side and design pressure of low pressure side. Moreover, this credit is subject to defining these check valves as Class-I (terminology used by Shell DEP) or safety critical element. You must mark it as safety critical element on your P&ID and include them in your safety critical devise register. That means, these check valves would require periodic preventive checks. 

 

Another point is, if your low pressure side corrected hydrotest pressure is within the high pressure side PAHH value, then you may negate the reverse flow scenario. However, this is subject various terms and conditions. Please refer to Shell DEP 80.45.10.11 / 80.45.10.10 or API-521. But I think in your case, this may not be applicable as gap between 600# to 150# system is significantly higher. 

 

Note that, check valves never considered a tight shut-off element and there will always be leakage through check valves. Hence, check valves never been allowed as a sole protection to negate reverse flow scenario. 

 

Hope this helps. 

Edited by Jiten_process, 13 September 2020 - 03:27 AM.

[mido]

Thanks every one for valuable comments ,as we cannot change psv size ,we will disconnect the two system by a blind.