8 replies to this topic

[farid.k]

Hye

 

Normally control valve that regulate flow will have common isolation valve, drain and bypass. Is it necessary to put that item also for ON-OFF valve which is triggered during emergency case?

I don’t think so since for me it is considered as safety item that shall be service during turn around only. Having bypass considered bypass the safety. Appreciate to advise me.

 

[sukanta87]

During start-up, the shutdown valve in closed position. The upstream of shutdown valve will be pressurized at high pressure (i.e 100 barg) whilst the shutdown valve may still kept at 2-3 barg. High differential pressure across the shutdown will results high torque against the actuator.

http://webwormcpt.bl...sdv-bypass.html

 

In emergency if shutdown valve is actuated, high DP will create (if there is no bypass or bypass is closed) which is bad for the SDV. Moreover, in many cases the SDV bypasses are smaller than the usual bypass size. Even the SDV bypass may equipped with RO (restriction orifice), it depends. After sometimes of operation, SDV bypass can be closed as your needs.

[fallah]

farid.k,

 

It depends to the on/off valve function. For an ESDV considering by pass isn't normally authorized due to safety issue, but for SDV it could be considered...

[farid.k]

farid.k,

 

It depends to the on/off valve function. For an ESDV considering by pass isn't normally authorized due to safety issue, but for SDV it could be considered.

what is the different between ESDV and SDV? act the new valve is to protect boiler. for me, it still consider as emergency shut down valve. appreciate to advise. 

[fallah]

farid.k,

 

There is no difference between them in construction and accessories standpoints...

 

SDVs are the valves will be actuated to be closed during partial or total plant shutdown of system being protected by them.

ESDVs are the valves will be actuated to be closed during emergency condition when triggered by ESD level signal and are normally located at incoming/outgoing lines of a plant or each fire zone inside a plant.

 

As can be seen an ESDV is more important than a SDV in safety standpoint....

Edited by fallah, 23 July 2014 - 02:42 AM.

[farid.k]

farid.k,

 

There is no difference between them in construction and accessories standpoints...

 

SDVs are the valves will be actuated to be closed during partial or total plant shutdown of system being protected by them.

ESDVs are the valves will be actuated to be closed during emergency condition when triggered by ESD level signal and are normally located at incoming/outgoing lines of a plant or each fire zone inside a plant.

 

As can be seen an ESDV is more important than a SDV in safety standpoint....

my concern is, if i put bypass, they have chance to bypass it and no more protection to the boiler during by pass. i also no point why should i put bypass. unless control valve that regulate flow that might need to service time to time

[fallah]

farid.k,

 

Normally by pass is to be considered for a control valve due to one of the following three reasons:

 

-Start up by pass

-Maintenance by pass

-Control by pass

 

If you have to use one of the above by pass and are worry about any safety issue, you can put it in locked closed position in normal operation condition...

[sanchit_11]

Dear Farid,

 

I really appreciate your concern with provisions of bypass across shutdown valve (ON/OFF). Please find point wise analysis for where you should installed bypass across shutdown valves.

• It is to be noted that any shutdown valve at whatever line it is installed does not have much pressure difference across it when it is in open condition but it will experience appreciable pressure difference only when valve will be in closed position.
• Actuator on shutdown valve will be sized on basis of torque required to overcome this pressure difference to open the valve again when shutdown reason is taken care of.
• If pressure difference across shutdown valve is more than size of actuator will be large and vice versa. But some time with large size valve which is having high pressure difference across them in closed condition actuator size required become very large.
• Thus in order to avoid usage of very large size actuator a very small size bypass normally 2" or 3" is installed across valve. This bypass equalizes upstream and downstream pressure and reduce differential pressure across valve and help in reducing size of actuator.

So, from above we can conclude that bypass of shutdown valve is not required unless reduction in size of actuator is required. Sometime Installation of large size actuators are not possible due to space restriction as at offshore platforms where space is the most costly item.

 

I hope this answer your query.

 

regards

Sanchit

[sukanta87]

Thanks sanchit for the clear view with elaboration.