4 replies to this topic

[jprocess]

Dear All,
What is the function of blowdown valves that sometimes designers
include them in parallel with psv(s)?
Also,sometimes designers include a balance line in parallel with that
blowdown valve.I think this should be because of high pressure
difference that exist up and downstream of blowdown valve.Am i right?
If so,is there any magnitude for that delta p that upper that value we
should consider balance line?
Thanks in advance.
Best of Luck.

[djack77494]

Blowdown valves on compressors are used typically to bleed off the pressure that remains trapped in the various parts of the system after the unit is shutdown.

[Nirav]

hello,

Blow down Valves (BDV) is a part of safety system of the unit or a section of unit.

Purpose of PSV is to release overpressure automatically when operator is unknown or not aware about the abnormal conditions which underwent before a corrective actions could be taken up.
Sometimes, after knowing the abnormal conditions, an operator is trying to reduce the possible scenario which could release material to flare or atmosphere. However, if corrective actions are not enough to reduce the contingency, PSV will pop.

Now, BDV is manual de-pressurization by operator when he wants to release pressure quickly before it reaches to abnormal conditions. The reason could be any. BDV can also operate 'automatically' based on safety system interlocks. Refer to Cause & Effect diagrams. You would come to know about this aspect.

The basic difference between PSV and BDV is the mode of operation. BDV is operated by "pneumatic" action (Instrument air). PSV is operated by mechanical action (Spring). Thus, PSV is independent of system failure. PSV is "ultimate" safeguard when things are going out of way and no one is able to control anything.

Now, a small line bypassing BDV. This is provided to de-pressure the equipment or section in a controlled manner (slow). This is not during abnormal conditions. But during normal shut down of the equipment. Generally, you don't de-pressure quickly during normal shutdown.

I hope above explanation helps ...
More opinions are always welcome

Thank you,

[jprocess]

Dear Nirav,
Thanks a lot for your valuable reply.
1.The role of bypass line can not be for making balance up and downstream of BDV when there could be high delta p?
2.The mode of operation for BDVs will also be the same(pneumatic),when they operate with interlocks?Also,could you please make a clear definition from interlock?
3.You stated that " psv is independent of system failure".What about pilot operated safety valves?
Thanks in advance.
Warm Regards.

[Nirav]

1.The role of bypass line can not be for making balance up and downstream of BDV when there could be high delta p?

I think your interpretation is based on your observations of by-pass valves around big isolation valves in process applications. For BDV, we dont' want to pressurize downstream system (e.g. flare header). But you need to understand fundamental aspect of your query. Any kind of pneumatic valve (control valve or BDV or ESDV), has "actuator" which is designed considering the maximum dP across the valve at any time. When you have high dP, relative size of actuator is higher compared to the valve having lower dP across it. In case of BDV, process datasheet must specify maximum upstream pressure and the downstream flare header pressure. Based on this values, actuator would be designed in such a way, that we don't need to pressurize downstream system before opening.

2.The mode of operation for BDVs will also be the same(pneumatic),when they operate with interlocks?Also,could you please make a clear definition from interlock?

Yes, during interlocks also, they operate based on pneumatic mode. The "cause & effect" diagrams are interlock diagrams. For example, consider a pump having suction vessel in upstream. A Low-Low level in vessel (a cause) will shut down (effect) the pump in order to prevent damage of a pump. Similarly, there are detailed interlocks for entire plant.
BDVs are sometimes provided based on "de-pressurization philosophy" and study in details. During safety interlock, a particular section of process would be entirely isolated by means of ESDV and then, BDV in that section will open to de-pressurize that section.

3.You stated that " psv is independent of system failure".What about pilot operated safety valves?

Pilot operated safety valves are also mechanical device. Instead of spring, it utilizes system operating pressure itself to hold the valve seat in its closed position. At top and bottom of valve seat, there is same pressure during normal operation. So, there is no resultant force on seat. As soon as pressure rises above set point, the top section above valve seat releases pressure to atmosphere or header (very small amount). So, now there is difference in upstream and downstream pressure of valve seat. Therefore, it opens as a +ve upstream pressure.

Again, read API-RP 520. Read free literatures on web about "pilot operated valve". Anderson Greenwood's website provided very good articles on it.

Thanks, and Best of luck