Flow Rate Vs Pressure Loss 4

[asade]

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.

 

[asade]

BigInch,

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

 

[asade]

BigInch and All,
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

 

[BigInch]

Looks good!
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.

 

[asade]

BigInch,
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

 

[BigInch]

BEP is the point under the maximum of the efficiency curve, which looks to be at the point defined by x = 2270 gpm, and y = 1410 ft. Red Star. That is the ideal operating point. The Efficiency curve is now the yellow line.

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.

 

[SNORGY]

Of course, installing the VFD in a motor of that size will require about 32 square feet of MCC cabinet area, soft start, harmonic filters, dV/dT filters, electrically insulated bearings and coupling, and maybe a few other things that you might not have yet.

 

[asade]

BigInch,

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

 

[asade]

BigInch,

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

 

[BigInch]

You'd still need to go lower with that rpm, until discharge pressure is <= 285 psig. Depending on specific gravity of the crude, maybe 700 feet of head or so.

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.

 

[asade]

So, the pump speed would be adjusted by VFD till we get desired pressure below the derated pipeline MAOP.

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

 

[BigInch]

You got it.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[BigInch]

Check to see if the existing motor will handle a VFD. Not all will do well with a VFD.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[asade]

Thanks all,
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

 

[GHartmann]

Just a cautionary note. Once you have decided on a new speed and head you need double check to see if the pump will deliver the capacity you need.