Back Pressure On Gas Export Line

Dear All

 

I appreciate the effort you all put into supporting young engineers who are new to the industrial world.

 

My company recently had a client who produces natural gas and exports via a 8" line to a recipient who utilize the gas for other purposes.

 

It has been observed that whenever the gas recipient temporarily shutoffs their reception of gas due to a temporary maintenance or for any reason whatsoever, there is usually a back pressure build up to the gas scrubber.

 

I have attached a sketch of the gas supply system below.

 

Although, the system was initially designed to flare once the export line is shutoff, but it seems the flare line and Control Valve  system is not able to do this. The pressure builds up from the shuf off hand valve (shown in the diagram) backwards to the scrubber.

 

The pressure downstream the scrubber (production separator) is 930psi, PCV is set to open at 1000psi, flare is atmospheric (1 atm), temperature downstream scrubber is 90 deg C. Gas flow rate is 0-30 MMSCFD.

 

I have been assigned by my boss to propose a possible rsolution to prevent the back pressure.

 

Some of my observation during my discussions with the client at the concluded site visit are as follows:

 

1. The export tie in point is a simple tee joint (I am surprised at this) which I believe should have been a valve manifold arrangement.

 

2.  PCV installation on the flare line might have not been the best judgement (my thought), I prefer to take my PCV reading just before the shut off hand valve where the pressure is expected to build up, but however this does not stop the back pressure.

 

3. There is also a massive hydrate formation buildup downstream the PCV bypass hand valve. (I am not sure why this has happened considering that the operating temperature of the gas is above the hydrate formation temperature)

 

I had earlier suggested installing a valve manifold at the export gas tie in point to check mate the back pressure. I felt a single check valve on the export line (upstream the hand valve) would be catastrophic due to pressure buildup in two direction. 

 

However, I am not sure that installing the valve manifold would be the best option in terms of cost and performance. Please could I get support as to other possible solutions / configurations that will checkmate the back flow to the scrubber..

 

Any form of support would be appreciated.

 

Please note that the attached picture represents the current configuration of the supply point at the gas production facility.

 

Thank you. 

Dear All

 

Please kindly ignore the 800 psi indicated upstream (before) the scrubber.

 

Thank you.

The controller is probably normally in a condition called "reset windup". So it takes a while for the output to begin opening the valve. All that is necessary is to modify the controller to eliminate the controller saturation. The valve probably should have a positioner, too

 

How do you know there is a hydrate?

 

Bobby

Thank you Bobby for your response.

Yes the valve has a positioner, it is a pneumatic valve.

We had a valve specialist check the valve but he didn't see any thing wrong with the valve.

Do you think the PCV position is ideal.

Also it appears the PCV does not notice the pressure build up/back pressure.


A report I received from a site visit conducted at the production facility indicated that there was hydrate on the PCV bypass.

Thank you.

You didn't read or understand my response above.

 

Bobby

dear emekar,

 

First please make it as a habbit to always give a good sketch of your system, such as what I have done. It takes only 15 minutes.

Second, refer to enclosed excel sheet for further explanation on your problem, it just a simple hydraulics understanding

 

 Back Pressure On Gas Export Line.xlsx   57.6KB   30 downloads

Edited by Erwin APRIANDI, 26 August 2019 - 03:24 AM.

Dear Bobby and Erwin

 

Apologies for the late response, I was away for a while.

 

Bobby,

 

I read your response again, I understand now and tried looking up reset windup and how controllers can be modified.

 

Thank you for your response, I appreciate it.

 

Erwin,

 

Thank you for the sketch will try to do so next time. 

 

Please I would like to repeat the question since it appears I didn't really do a good job the last time to communicate the problem effectively.

 

So Again: The export line is sometimes closed for routine maintenance at the other end of the line by the customer. This usually results in a build up of pressure on the gas producer side (from the export line to the scrubber shown on the diagram), especially on the export line (which is now closed at the customer end). The PCV is set to fully open at 1000 psig but does not fully open at the set as expected. Hence there is back pressure up to the scrubber.

 

In order to mitigate the back pressure, the PCV was changed by the producer (which is the client), this was before our company was contacted, they felt the PCV was faulty.  

 

My company was recently contacted to confirm if the back pressure will build up again if the customer decides to halt receipt of gas at their end for routine maintenance.

 

 

During our recent site visit to the client site (Which I wasn't part of the visiting team), the operators reported constant hydrate buildup downstream the PCV before it was changed.

 

I suspect this is due to the massive drop in pressure from around 930 psi to atmospheric conditions (although I am yet to calculate the hydrate formation temperature). Oulet of gas scrubber is around 80-90 deg C

 

 

I intend to suggest installing a gas fired heater to preheat the scrubber outlet gas to around say 120 deg C, I believe this would solve the problem. The heater would use a portion of the natural gas sent to flare as fuel gas but I am unsure where the best point for a fuel gas tie-in on the line is based on the diagram.

 

Please kindly advise if this is the best option to prevent hydrate formation downstream the PCV.

 

I will also propose Bobby's solution to the team so we can check the new PCV controller for any possible modification. 

 

Please I rarely have any industrial experience or sound knowledge of instrumentation so I apologize if my questions sound unintelligent.

 

Thank you.

How did the operators determine the presence of a hydrate? They couldn't look inside the pipe. Is this a new problem?

 

Bobby

Dear Bobby

 

Thanks for your response.

 

1. I was not part of the site visit team, I only read the report they sent to us. The facility superintendent reported the formation of small icy crystals (hydrate) down stream the bypass valve (and PCV). The site visit team did not bother to ask how he knew they just recorded. I guess since he is the facility superintendent he would have probably noticed it during the maintenance.

 

2.The hydrate formation problem is not necessarily a new problem. being that the client had initially attributed the formation of hydrate and the massive pressure drop across the PCV as the root cause of the PCV not responding as it should. hence was changed it the first time (That is before my firm was contacted).

 

Please just a correction from the previous post the scrubber outlet temperature is at 80F (and not DegC). This corresponds to 27degC.

 

I did a very preliminary simulation showing the PCV inlet to the KOD outlet.

 

INPUT:

 

PCV inlet pressure= 930psig,   PCV Outlet Pressure= 0 psig, Inlet Temperature: 27 degC

 

Gas Composition

 

C1      0.7162

C2      0.1141

C3      0.0797

I-C4    0.023

N-C4  0.0207

I-C5    0.007

N-C5   0.0039

C6       0.0042

C7       0.0016

N02      0.0066

CO2      0.023

 

RESULTS:

 

PCV Outlet Temperature: -32 degC

 

Hydrate Formation Temperature: -31 degC

 

Outlet Vapour Phase Fraction: 0.98

 

 

The results suggests a possibility for hydrate formation.

 

The current PCV is Emerson Fisher V 500 globe control valve with a 3610J positioner and a 2052 actuator and a gauge pressure controller.

 

Please any suggestion would be appreciated especially as regards the best option to prevent hydrate and back pressure.

 

Thank you.

The pressure of the vent gas is not 0 psig until it exits the flare tip. Sounds like you have not done a hydraulic analysis. And, if the PCV is set at 1000 psig, the backpressure at the scrubber will be > 1000 psig.

 

I would expect to see what might appear to be crystals when venting this gas whether there is hydrate or not. The usual solution to prevent hydrates when reducing the pressure of a wet gas is to inject methanol in the high pressure stream. Your stream would not require much.

 

Even if hydrates are forming downstream of the PCV, I do not expect them to "stick". So there is much missing from your dialog. And it seems you have done little analysis of the situation. Troubleshooters need lots of information, which we lack. We don't know if this is a recent occurrence, or is chronic.

 

Bobby