5 replies to this topic

[asade abiodun]

Good day All,

Please, I need insight on how to achieve a low pressure at a pipeline tie-in point. The case at hand is stated below:
A crude oil pipeline pump is to transport a 70000 bopd from an offshore storage facility through a departing pipeline and tie-in to another pipeline which has been derated to 285 psig. Based on the pump performance curve, the discharge pressure from the oil pipeline pump is 526 psig which is greater than the MAWP of the tie-in pipeline.
The oil flow rate is to be maintained while the delivery pressure at the tie-point should not exceed the MAWP of the pipeline which is 285 psig. Is it possible to maintain the flow rate at low pressure at the tie-point?

I developed a 2 case models using PIPEPHASE software. The cases are to see the effect of installing a PCV to regulate the downstream pressure and without a PCV.

Case 1: Regulating the downstream P by setting the PCV @200 psig gives a delivery pressure of 173.6 at the tie-point. Discharge pressure from the pump is 553.2 psig with pump power of 787.6 hp and 0.85 efficiency.

Case 2: The delivery pressure at the tie-in point is 526.3 psig without any PCV installed downstream on the oil pipeline pump discharge line.

Please, I need your comments on the two results I provided.

Also, a colleague suggested that we should install a PSV on the departing pipeline to provide pressure relief should the pressure goes above the required downstream tie-in pressure. The departing pipeline MAWP can withstand any excess pressure that could be generated from the pump. Is it logical to install a PCV on the line with the aim to control the tie-in pont pressure which is some kilometers away?

Thanks for your anticipated comments.

Edited by asade abiodun, 02 August 2012 - 09:56 AM.

[TS1979]

1, Obviously, case 2 is unacceptable because it will overpressure the downstream piping.
2, Case 1 will work. PSV is required for case 1 in case the pressure control valve fails.
3. There is another option is to trim the pump impeller and/or install a VFD for the pump

See attached sketch for more information.

Attached Files

•  Crude Oil Transfer.pdf   166.98KB   66 downloads

[asade abiodun]

Thank you TS1979,

You have opened my eye to other options. I appreciate.
Please, what is a VFD?

It was the case of PCV failure that makes us consider the installation of PSV on the departing pipeline from the platform as an option to prevent the downstream tie-in point connection. But according to API 14C, you do not need to install a PSV on the pipeline when the "pipeline has a MAOP greater than the maximum pressure of any input source". In this case, it is the shut-in head of the pump.

My view is that should the PCV fails and the PSV relieves excess pressure from the pipeline, we wouldn't get the rated oil flow rate at the tie-in point. Therefore, we would be left with oil pipeline pump shutdown to prevent excessive overpressure.

What is your opinion if the installation of a bypass line with Self-regulating Pressure Valve is proposed which would come onstream when the two PCV fails. Would these be a good option as against capital cost?

Edited by asade abiodun, 03 August 2012 - 03:02 AM.

[paulhorth]

Asade,

I agree with the post from TS 1979. I would add that, as well as a PSV, you should fit a high presure trip (PSHH) to the pump discharge, acting to stop the pump. You will then have two independent means of protection against overpressure as specified by API RP 14C.
Your comment about API RP 14C not requiring a PSV does not apply to this situation, because your pump is connected to a pipeline with a lower MAOP than the pump shutoff. The length of pipeline rated for the higher pressure does not matter - imagine this being only 1m long and this becomes obvious.
Your calculated pressure drop to the tie-in is only 27 psi (200 - 173). Why then are you using a pump which delivers a much higher pressure? The pump should be replaced or you will be burning more than half the power across the control valve.
I don't understand what you intend by the self regulating bypass valve. Forget this, and install the PCV, PSV and PSHH, and you will have a safe system complying with international codes.
You need to think carefully about where the PSV discharge is routed to.

Paul

[asade abiodun]

paulhorth,

Thanks for your input. The pump was has been installed and operated before the facility shutdown. During this period, the main oil pieline, which is the tie-in point as been derated to a lower pressure.

Yea, I agreed that the pump can be changed to a lower head pump or change the impeller size while maintaining the crude oil flow rate. But, a new dimension has been added to the study by the information I received from the Company's Operation Personnel.

We were told that they can achieve pressure drop across the crude pipeline without reducing the crude oil flow rate to arrive at the tie-in point using Drag Reducing Agent (DRA).

I am curious. Please, I would appreciate if anyone can explain to me how this chemical works. What is the injection rate into the pipeline?

Thanks.

[TS1979]

VFD - Variable speed drive. If you run the pump slower, the kinetic energy of the liquid will be lower and the lower pump head will be converted.