Flow Rate Vs Pressure Loss
Flow Rate Vs Pressure Loss
(OP)
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 psig 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.
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 psig 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.





RE: Flow Rate Vs Pressure Loss
I assume by a pumping power stated as 787.6 psig you meant to say 787.6 hp.
With respect to the second question, it is common to have pressure control and over-pressure protection in a pipeline, each separate from the other. In fact, in Canada, it is a pipeline code requirement, although the over-pressure protection does not necessarily need to take the form of a relief valve. I am not familiar with B31.4 & B31.8 but I would suspect the requirements are similar.
RE: Flow Rate Vs Pressure Loss
There are times that it makes sense to leave an over-powered pump in service and dump excess pressure across a pressure regulator. For example, if a replacement pump has too long a lead time or the pressure requirement is scheduled to change later, etc. All things considered it may make economic sense to follow SNORGY's recommendation and get the right pump. It may not.
Rules of thumb are a crappy substitute for Engineering analysis in either case.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Flow Rate Vs Pressure Loss
On the pump power, I meant 787.6 hp not "787.6 psig".
The oil pipeline pump is an existing pump. The challenges occurred because the tie-in pipeline was derated to lower pressure when the production facility was shut-down.
RE: Flow Rate Vs Pressure Loss
High pressure kill switches and other forms of "electronic pressure control", including the so-called High Integrity Protection Systems, "HIPS", while often found in chemical process plants, are not considered a valid mode of pressure control in pipeline work. Pipeline codes generally do not recognize kill switches as a form of overpressure protection (in case the discharge PCV fails), so a hard-piped PSV is needed. Many companies have HIPS prohibitions specifically written into their pipeline design standards as well, Saudi Aramco being one I can name in particular.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
The PCV is installed at the pump discharge. The pump is located on an offshore platform.
Yes, I agreed with you on the fact that the PCV can fail though another PCV bypass connection was designed. 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 I proposed the installation of a bypass line with Self-regulating Pressure Valve that would come onstream when the two PCV fails. Would these be a good option as against capital cost?
RE: Flow Rate Vs Pressure Loss
I received an information from the Client Operation Personnel that they can achieve pressure drop across the crude pipeline without reducing the crude oil flow rate to arrive at the tie-in point to derated main oil line by 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 rate of injection into the pipeline?
Thanks.
RE: Flow Rate Vs Pressure Loss
RE: Flow Rate Vs Pressure Loss
But, back to the relief valve. You are putting in the relief valve to protect the derated downstream portion of the pipeline from overpressure, not to protect the "good" pipeline coming from the platform.
Perhaps you also need to look at what sets the derated pipeline pressure at it's destination and what is needed there to keep from overpressuring this line.
Remember the pressure in the "derated" pipe is set by its friction loss and destination pressure and not by the pump that feeds it.
However, with a blocked in flow you can certainly overpressure the downstream pipeline and it must be protected against this overpressure.
Good luck.
RE: Flow Rate Vs Pressure Loss
Now I have to come down a bit hard on you, but hopefully you will thank me for it some day. It would seem that you have a lot to learn about interpreting the intent of code requirements.
If the downstream pipeline closes a valve anywhere between the tie-in and the beach or downstream plant, pressure can back up to its MAOP and continue backing up straight into your pump discharge. When that happens, your pump will keep pressurizing the whole downstream system. Then we'll have another Maconda out there. Not good. A flow diversion is not an acceptable alternative, because when that downstream pipeline valve closes, the pressure will still back up all the way to the pump discharge before your bypass opens. When the bypass opens it will only keep the pump from adding more pressure to the downstream pipeline. It will not reduce the pressure in the now overpressured downstream pipelines, unless all that downstream pipeline pressure spills back through your now open bypass and floods the platform. You need the PSV and an ESD and the PSH and whatever else that is required by API14C. If you don't put it there, you WILL NOT GET THE PERMIT TO OPERATE IT. So, you see... I win.
You will do much better in this business if you just read and learn word for word what the codes say and do it exactly like they say, rather than make up &^(*&% stuff to justify, for who knows what reason, why you should want to give your client a potentially dangerous design that does not comply with the code and risks your personal career to boot. What's your motive?
Sorry for the hammering, but I think its better to get it now from me now than from your boss, or worse, your client.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
BigInch,
Thanks for the correction. I accept in good faith cause we learn daily and wanted to be a better engineer.
I agreed to the installation of PSV on the pipeline in my second comment. May I point out that PSHH has been designed to be installed on the pipeline in accordance with API 14C.
When I suggested the bypass line with Presure Regulating Valve, I didn't consider the scenario of sudden closure of downstream pipeline valve which would cause pressure surge in the pipeline. I was ONLY looking at the PCV failure. Now I know that I must consider on all sides in making my good engineering judgement. Thanks again.
What about the information I received regarding the usage of Drag Reducing Agent? Do you have any understanding about the chemical usage?
RE: Flow Rate Vs Pressure Loss
The drag reducing agents can considerably reduce differential pressure across the pipeline (inlet P - outlet P) for a wide range of flowrates, thus you may be able to lower the required inlet pressure necessary to move your design flowrate. The drag reduction agents can be expensive, but when a lower differential pressure solution is absolutely necessary, they offer a solution. If you can run drag reducers and then lower the shutoff head of the pump to the lowest MAOP of the two pipelines (reduce the impeller size, or limit the rpm), you could potentially do away with the control valve and PSV, although the PSH to close the ESD and initiate pump shutdown would still be the recommended solution.
If you do not have a VFD there at that pump, keep the control valve idea in case you do need to control flow or pressure for operating purposes during shutdowns, startups and various other scenarios. Running a pipeline with only an on/off pump control can be a pretty limiting condition in all but the shortest pipelines. Even in a short pipeline, pump starts/stops into an open system can sometimes result in hi pressure transients echoing quickly back and forth down the short line. The CV is a good thing, if you have a chance to keep it in the design, hold on to it.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
Please, what is the best position that I can position the PSV discharge outlet connection?
I have two positions on my mind but with different challenges. They are:
1) To discharge directly into the atmosphere. This could be a problem because of enviromental pollution.
2) To the surge tank. The problem here is that the tank is designed at 1.5 psig as noticed on the Surge Tank P&ID which is below the relieving pressure from the PSV. The set point for the PSV is @200 psig.
I need your advice.
Thanks for your anticipated comment.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
You stated earlier that you wanted the PCV to control the downstream pressure at 200 psig, therefore the PSV should be set higher than the PCV outlet pressure.
What you are trying to protect is the downstream MAOP of 285 psig. Thus you could set the PSV at 285 psig.
If your surge tank is at the suction of the HP pumps, then you are probably OK venting back to this tank (providing you have sufficient volume). In this case you are merely recirculating your fluid back to the tank.
As long as there is no flashing vapor created, then going back to the tank with "returned" liquour shouldn't overpressure the tank.
RE: Flow Rate Vs Pressure Loss
You must go to a tank. Definitely not atmosphere. The tank vent must be sized so that the filling rate of the tank, at the PSV discharge rate, will not create any overpressure.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
rmw
RE: Flow Rate Vs Pressure Loss
rmw, I think you must have caught me out a time or two. I have the occasional lapses. I fact, I had to apologize for getting something wrong just last week.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
A good teacher could be hard to pass his/her message, and draw the protege near with affection to show that he/she meant well for his/her upkeep. I understand you because my mum was a teacher. Thanks for making some us to tow the line of good engineering principles and judgement.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
I gave the following as my recommendation on the problem we discussed above:
In the event of the PCVs failure, the following recommendations are advised:
1. PSV/PRV should be installed to protect the downstream derated pipeline from overpressure with its set point at 10% higher than the lowest maximum allowable operating pressure (MAOP) of the downstream pipeline.
2. The PSV discharge line should be routed to a safe tank with adequate capacity.
3. The installed PSH should close the SDV and initiate pipeline pump shutdown.
To maintain 70,000 BOPD through the pipeline pump and achieve low delivery pressure at the tie-in point end on the derated downstream pipeline, the following are recommended:
1. Install a variable frequency drive (VFD) for the pump if possible to adjust the pump speed (rpm). This would enable the pump to run slower and discharge at a lower head.
2. Change or trim the impeller the impeller diameter.
Are my recommendations adequate enough? Kindly review and comment. I highly appreciate your inputs.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
Check this when you can.
The drop in flowrate when using a VFD is directly proportional to speed, so find your required speed based on pump BEP flow as it is now and whatever will be your desired flowrate.
New_speed = New_Flow / BEP_flow * Rated_speed
Then check that pump head at that new speed will be sufficient to get that flowrate down the pipelines. New_Head = Head at rated speed * New_speed^2 / Rated_speed^2
In order to use a VFD, that New_Head should be >= head required to flow down the pipeline.
Othewise, keep the same speed and try changing the impeller diameter to the required head at the new flowrate.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
Please, how can I determine the BEP point on this pump curve? I am used to the curve with different efficiencies in different curve.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
It looks like the former design point, maybe where some previous system curve crossed the head line was near 2010 gpm and 1500 ft., but for whatever reason, some engineer thought it would operate at, maybe, the pump was initially selected for 1755 gpm and 1588 ft.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
RE: Flow Rate Vs Pressure Loss
Thanks for the explanation
The pump is been refurbished to handle the new flow rate of 2042 gpm and 1520 ft due to increase in crude oil production. The pump was initially selected for 1755 gpm and 1588 ft. So, can i say that the new operating condition is below the BEP of the pump? If yes, what is the technical implication of this on the pump performance?
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
Based on the information provided above on VFD, I determined the new speed at 800.49 RPM and new head at 481 psig.
In my simulation, I input the new head for the pump and the result I obtained shows that the crude oil would arrive at a pressure above the derated pipeline MAOP. Therefore, would I be right if I remove VFD installation from my recommendation and advised ONLY the trimming/reducing of the pump diameter.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
Even with a VFD, efficiency will start to be lost <=50% rated rpm, but it may still be OK.
You won't be able to trim the impeller that much. Generally not possible outside of a 10% change in D.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
If by trimming the pump impeller the desired pressure is still not achieved, then VFD would be the final recommendation OR outright purchase of a new pump. Am I correct?
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Flow Rate Vs Pressure Loss
I am highly delighted and grateful for the knowledge I had gained from this forum on this matter.
I am what I am by His grace
RE: Flow Rate Vs Pressure Loss