12 replies to this topic

[Fr3dd]

Good Morning ChE Community,

I have a doubt and I hope somebody could throw me some light on this, I would really appreciate it.

First, a quick overview of the situation: It's a Gas transport pipeline (natural gas) with a pig launcher. The connection to the pig launcher is isolated by two massive motorized ball valves (48"), each one with its corresponding drain and bleeder connection for draining and bleed the valve cavity (please correct me if I'm wrong in this).

Here's the thing, we've been asked to connect the bleeder valves to the flare header, so the venting procedure will be performed via flare; but, additionally, they are asking me to connect the bleeder valve also to the main gas line in order to "pressurize" the cavity before valve operation (open/close).

Is this procedure normal?. I'm not familiar with the operation of ball valves of this size but, is it really necessary to "pressurize" the cavity before operating the valve?.

[TS1979]

It is difficult to say if the pressurization of the valve body is necessary or not. For the big size valve, the operating procedure is depending on the valve structure. Valve supplier is the best source to get this information.

[Fr3dd]

Thanks for your reply TS1979,

I'll take your advice and see if I can find any info from the supplier. Thank you!.

[fallah]

Here's the thing, we've been asked to connect the bleeder valves to the flare header, so the venting procedure will be performed via flare; but, additionally, they are asking me to connect the bleeder valve also to the main gas line in order to "pressurize" the cavity before valve operation (open/close).

Fr3dd,

You are asked by whom? They are asking you to connect the bleeder valves to both flare header and main gas line?

Please ask them to provide thier request more clearly; also ask them to clarify how to pressurize the valve cavity by such a connection while the cavity is isolated from the main line in closed position and would be as a part of the main line in open position.

Fallah

[Dacs]

Without the benefit of a detailed P&ID, I assume that somone who gave the order (client maybe?) wants to have a pressurization line from the line upstream of valve to pressurize the section downstream to facilitate easier valve operation (how much upstream pressure are we talking about?).

I assume that they want to have that to reduce the torque needed to operate the valve, especially for that size.

What I would do in your place is to have the valve vendor calculate the the torque needed to operate the valve (given your pipeline conditions) and if it exceeds the available torque provided by the actuator motor, then you can consider placing the pressurization line.

Then again, the valve vendor is in the best position to decide if that is needed or not.

As per single line going to flare and pressurization line, I guess you have to check how to isolate both lines (by placing blinds?) in accordance with your company/client specs.

Hope this helps

[Fr3dd]

Hello guys,


Fallah,


I think my explanation may be confusing. I'm attaching a simple diagram of the situation. Bleeder valves from the ball valve cavities will be connected to flare for depressurizing purposes and also to the production line for cavity "pressurization".


Dacs,


The comment was made by the client indeed. To ease the valve operation could certainly be a reason. I'm going to ask if the torque calculations are already done in order to have a justification for this requirement. Now, about the isolation of the flare lines, the scheme shown in the attached image is our first approach (the new connection is highlighted in blue). Do you think that additional isolation measures should be taken?


I really appreciate the time you took to answer. Many Thanks!.

Attached Files

•  Pressurizing connections.JPG   17.22KB   30 downloads

Edited by Fr3dd, 13 August 2012 - 06:21 AM.

[fallah]

Fr3dd,

Thanks for uploading the sketch. Now the matter is more clarified.

Venting the cavity to the flare while the valves are to be closed can be considered as a normal procedure to prevent valve's seat/internal parts to be longly under such high pressure.

About pressurizing the valve cavity before opening in order to actvalte pig launcher, IMO seems such high volume to be pressurized by small size connection to reduce the valve torque and also prevent the valve internals suffering such sudden high pressure from downstream side.

Fallah

[Dacs]

First of all, I don't see the use of a PSV (I assume, but it also looks like a needle valve) in the line going to flare. My understanding is that line is there to purge the valve cavity of process fluid.

Second of all, the equalization tapoff provided on the rightmost motorized ball valve does not make any sense because it looks like it's tapping to the high pressure part. You want to have the tapping point on the low pressure side.

I attached what I have in mind. There are two schemes that I presented. Scheme A would entail isolation via blinds (blind is normally in closed position when pigging is not used). This is to ensure leakages are avoided but at the expense of operability (operators have to swing the blind, etc). Scheme B wouldn't have blinds and I placed a check valve on the line going to flare to avoid backflow from the flare network.

I added a PG on both valves for the operator to check the pressure in between MOVs. Normally you'd open the equalization line, the operator will check the PG, if the pressure is sufficient, open the leftmost MOV and then open the rightmost MOV.

I also added another equalization line to the upstream of leftmost MOV for provision to pressurize the upstream line.

I think 3/4 to 1" size for the flare and equalization lines is enough to accomplish the task.

I believe both will work well, it's up to the client if the valve arrangement is inline with their operations/maintenance philosophy.

Hope this helps

Attached Files

•  scheme.JPG   17.22KB   26 downloads

Edited by Dacs, 13 August 2012 - 07:15 PM.

[Fr3dd]

Thank you all for your responses!

Fallah,

You have a good point there; the connection was originally considered as a bleeder of the valve cavity. If it's going to be used to pressurize,some hydraulic calculation should be performed to check if the diameter is enough. I'll verify this.

Dacs,

The valve in the drawing is not a PSV but an angle valve, during depressurizing it is used to choke the flow (opening slowly); sorry if the drawing was confusing.

I don't know if I'm misunderstanding your point here; but the connection shown on my drawing is for pressurization of the cavity of the valve body (not the line between both of them). This is why both lines are tapped to the motorized valve body instead of the line.

Taking this into account, do you still think that this "pressurization" has something to do with the torque and the ease of operation of the valve?

About the equalization line, according to the procedure, before pigging, the launcher will be pressurized (using other connections not shown in my drawing) and with the left motorized valve open. By doing this, an equalization bypass is not necessary.

About the check valve in the line going to flare I agree with this but I also have a question:

When venting, pressurizing connection valves will be closed; also, when pressurizing, venting valves will be closed. All this valves (with the exception of the UZ valve) are manually operated since the pigging procedure is not a normal operating scenario but a maintenance action to be performed following a procedure: Do I still need the check valve or the isolation given by the manual valves is enough?

Thank you again for spending some time in helping me.

Regards,

[Dacs]

The valve in the drawing is not a PSV but an angle valve, during depressurizing it is used to choke the flow (opening slowly); sorry if the drawing was confusing.

I had angle valve in mind but I dunno why I typed needle valve, but anyway I don't object in having one, though I don't really see any reason to obtain a controlled flow when venting to flare, unless you're trying to limit the throttling to minimize sudden temperature drop.

I don't know if I'm misunderstanding your point here; but the connection shown on my drawing is for pressurization of the cavity of the valve body (not the line between both of them). This is why both lines are tapped to the motorized valve body instead of the line.

In the valve you'd have one exposed to higher pressure and the other one exposed to lower pressure. In your drawing, the bleeder tapoff seems to be located at the higher pressure side, which will make the arrangement moot since you can't equalize the pressure at both sides of the valve.

I guess you have to confirm if the bleeder is exposed to which side.

Taking this into account, do you still think that this "pressurization" has something to do with the torque and the ease of operation of the valve?

I can think of two scenarios of placing a provision for equalization line: one is for warm up and one is to reduce the pressure delta to facilitate valve opening. The latter applies with your scenario.

About the equalization line, according to the procedure, before pigging, the launcher will be pressurized (using other connections not shown in my drawing) and with the left motorized valve open. By doing this, an equalization bypass is not necessary.

I agree. No need for that extra line if you have provision to pressurize the pig launcher.

About the check valve in the line going to flare I agree with this but I also have a question:

When venting, pressurizing connection valves will be closed; also, when pressurizing, venting valves will be closed. All this valves (with the exception of the UZ valve) are manually operated since the pigging procedure is not a normal operating scenario but a maintenance action to be performed following a procedure: Do I still need the check valve or the isolation given by the manual valves is enough?

Your flare might have minute trace of acid gases (from other units) and if this is the case, you'd want to reduce any chance of those gases migrating to your line. It may be possible that you'd do the pigging with the flare backpressure at lowest (no relieving from other units) and suddenly there's a relieving incident in other units (which increases the flare backpressure) that may contain acid gases.

In this case, it might migrate to your line and I'm sure that you'll have pockets in that line and it may cause condensation (and corrosion).

It's just personal preference and I think you can do without it (the check valve and the blinds). To be honest, it may be over the top in your scenario, but I'm just coming from my experience dealing with acid gases and we tend to be on the conservative side especially when dealing with corrosion.

Edited by Dacs, 14 August 2012 - 07:46 AM.

[Fr3dd]

Thank you very much guys,

All of these tips were very useful. I have some things to check and consider but I think I'm on the right path now.

Thanks.

[Fr3dd]

Dear friends of Cheresources,

I wanted to post here the results of my initial query about the ball valves venting connections in order to help other people with similar issues in the future.

This are large ball valves which will isolate a pig station (usually closed/depressurized). Apparently, during pressurization/pigging operation, when these valves will need to be opened, there will be a big pressure difference between the valve cavity and the upstream (or downstream) line, depending on the case. This pressure difference is harmfull to the valve, so a pressurization connection for the valve cavities is needed in order to reduce this difference every time the valve is operated.

For cavity depressurizing, normally could be eough to put the valve in an itermediate position (neither open nor closed) in order to give some time to the gas in the cavity to vent: but, since this valve will have a two-position actuator (on-off) this is nor possible, so venting connections are required. Also, a regular vent to a safe location is enough, but in this case a venting to flare is required since we have to deal with noise regulations, since the installation will be close to an operations office.

All the additional measures to be taken (provision of a pressure transmitter, checking of vent line hydraulics, ...) were really useful. Thanks a lot.

Regards,

[Art Montemayor]

Fr3dd:

I have been involved in various pig station designs and my interest is primarily concerned with the safety aspect – as I hope it is with everyone who is reading this.

I consider the design, operation, and maintenance of a pigging station as one of the most important and potentially hazardous in a hydrocarbon production facilities. And because of this, I would ask for your comments on some points that I believe are related to this operation and the need to vent the block valve cavities in order to add them to my knowledge on pigging stations:

You stated “(the) pressure difference is harmful to the valve, so a pressurization connection for the valve cavities is needed in order to reduce this difference every time the valve is operated”. What is the brand (make) of the subject valve and its model number? What specific kind of potential damage is expected due to the pressure difference? What type and material of seats are employed for the balls? Are the venting precautions for the valve recommended by the valve fabricator?

Are you employing a safety-designed, “KEYED” operation of the pigging station? If not, what safety method(s) are you employing to ensure that the strict and regulated procedure for all related, step-wise valve operations always be done in a designed, proven, specific manner and sequence? Are the related pressure and limit switch indications and alarms being done and recorded locally and/or at a remote control station? Is the operation of the pigging station and the recommended venting operations stringently supervised on site every time the station is operated?

Thank you for your information and the anticipated feed back.