Booster pumps fluctuating flow 3

[devaxrayz]

Hi all,

I'm planning to install a booster pump in the middle of pipeline to transport (increase pressure) of crude oil from some treating station to shipping station.

The pump capacity is 50000 BPD at 250 psig. The oil flowing in the pipeline is normally also 50000 BPD, but one of the treating station is off, the flow can be drop to 40000 BPD.

Observing the pump curve, at lower flow rate more head is developed. While using system curve, at reduced flowrate lower head is required. So there will be a "head gap" between pump curve and system curve isnt it??

I'm not experienced on this so i can't not figured what will happen to the pump?? I'm guessing of pump surging.

If pump surging is happen i'm considering of these three alternatives to maintain the pump flow stability:
1. install a tank at discharge treating stations and use the booster pumps to suct the tank volume to shipping stations...(expensive i tought...)

2. add a recirculation line arround the booster pump with flow control valve at return line....

3. use MVD (motor variable drive) to match the speed with the system curve...

which is the best arrangement do you think??

thanx a lot for your comment........

-Rayz-

 

[Ashereng]

Rayz,

A control valve can be used to take up the difference between your pump curve and system curve.

Add a valve into your line between the pump and destination.

You didn't mention line size or distance, but guessing from your 250 psig discharge, I would guess a 16" line?

16" globe, complete with stuff, would probably run around $80,000 - $100,000 CDN.


I am guessing that you are using a centrifugal booster pump. I don't think surging is the issue - you are dropping 20% of flow, and presumably, over time. The only issue is excess head/pressure. For that, a valve should work.

 

[quark]

It is the other way round. The flowrate is less when the resistance is high. You should always keep in mind that the pump runs at the intersection of pump curve and corrected system curve. Suppose, the resistance in the system at 50000 BPD is 250psig, and you choose a pump with BEP at 50000 BPD and 250psig, the operating point of the pump is 50000 BPD and 250psig.

Now, you create some partial restriction to the flow and the pump has to work against increased system resistance and so the discharge gets reduced.

Now the question is what happens if the generation rate is lower than the pumping rate? There will be air pockets in the system all over. So, option 2 is not of use. Better is to go with option 3 but the with the reduced speed (to maintain 40000 bpd) the pump can generate a maximum head of 160psig (against the original 250psig). This doesn't create problem if your system resistance is purely dynamic.

Can you go with two pumps as another option?

 

[devaxrayz]

Ashereng & quark...

thanx for the quick reply...

I'm talking about reduced flow to 40k BPD which can relieved again to 50k BPD if the treating station is back to operation.

however, in point 2, i'm intended to install the flow control valve in recirculation line, not in the discharge line... the purpose is when the flow at suction is reduced, the valve will be open and add back some flow to the suction.

Due to that the cv will not be 16" big, but maybe 6" is sufficient...

Using CV in discharge line?? i still can not see how it will be works... altough i agree that it will fix the system curve. but how to control it?? "Ashereng" if you mean by pressure control, i dont think it could be happen since the system is purely dynamic (as quark said)

MVD although seem a good one, it wasnt a familiar device in here... so maybe it wont be favourable. So i still looking forward for the CV.

thanx alot

-Rayz-

 

[Ashereng]

Since you know what your flow rate is, you will also know what pressure you need (at your pressure sensor) to reach your destination (at the required destination pressure). The pressure you need would of course vary with flow. And, you said that you have the flow rates - 50000 BPD to 40000 BPD.

The pressure control valve maintains the required pressure at the pressure element.

 

[Ashereng]

It is the other way round. The flowrate is less when the resistance is high. You should always keep in mind that the pump runs at the intersection of pump curve and corrected system curve.

quark, I believe we are saying the same thing.

At 50000 bpd, the pressure differeence between the pump curve and the system curve will be less than at 40000 bpd. The pump curve is coming down and the system curve is coming up the chart.

Without something to "eat up" the excess pressure, that pressure will be propagated to the destination. If the destination can not accept higher pressures, then the pressure to the destination needs to be reduced. This is achievable at either the source of the pressure (pump), or in the line (valve).

 

[bulkhandling]

Talking about the preferred Option 2 by Rayz.

You need to check your system curve against you pump curve. The recirculation rate will not be 10000 BPD but much more than that. You have to pump more than 50000 BPD to compensate the lower pressure due to lower flow in the main pipe and you have to check if the total flow is still within the pump curve without running out.

And since it is continous operation, you'd better to keep the operating point within preferred range (=<110% of BEP).

Having said that, you should check your recirculation pipe size again with the new flow rate. Also the motor BHP shall be checked and see if energy consumption is much higher (even if the motor size is big enough) than the other options, this option may not be economically feasible.

 

[quark]

I totally agree with bulkhandling. It is a wastage of energy to recirculate the fluid back. Considering purely dyanamic head, either you have to run the pump at full speed or 80% of the full speed at the two given conditions and still operate the pump nearer to the BEP (if the initial design is at BEP). At 80% of the speed, there is an approximate reduction of pump head capacity by 90psig. If you can make two speed arrangement for the prime mover, you will end up cheap in investment and operation as well.

In any case automating the process to run the pump seems to be difficult to me. But we have wiser souls here. Let's hope for a better suggestion.

 

[devaxrayz]

Ashereng, since I already know the flowrate, it is better i use the FCV, whether using PCV and calculate the pressure i required at sensor point. But either using PCV or FCV, it means that the system will not fully automatic controlled. Operator will have to change the pressure/ flow setting everytime the flow changing -which is sadly, unpredicted and at some source are not good measured-

bulkhandling posting also make me realize that my prefered option can not fully automatic also(you're right quark...) Anticipating that the flow may can varied other than 40,000 and 50,000 surely make this more difficult )

I think that maybe MVD will done better...

Thanks all for the posting

-Rayz-
"this forum rocks..."

 

[bulkhandling]

Both MVD and control valve can do the job. MVD normally requires higher initial investment but saves energy when pumping lower flow. You may need to do a simple trade off study.

When talk about the automatic control of the process, I'm curious about the control of the primary pumps. Do all the primary pumps have the same owner? How do they control the flow accurately from each station?

In any case, (except that you add a surge tank as the pump box of the booster pump), you need to consider the control and calculate the head together with the primary pumps. Lower flow in the main piping before the booster pump also results in a lower friction loss.

Since there are only two different flows, you can accurately predict the pump speed (for MVD) or valve opening (for control valve, use Kv or Cv data from the valve manufacturer). So, when one of the primary pump stops of starts, you just need to switch the setting of the booster pump speed (or control valve opening).

 

[devaxrayz]

Primary pumps (pumps in treating station) is not owned by the same owner...different owner for different station.

How do they control the flow accurately from each station? I doubt they control and measured it accurately. That's why i'm affraid that actually the flow can go other than two flow i state before. So flexible control is necessary more than only two alternating setting MVD or CV.

bulkhandling...you were right, i also thinking that this case needs detail hydraulic analysis to get good result in whatever solution i choose (except if i choose to install the tank of course )

cheers,

-Rayz-

 

[Ashereng]

Operator will have to change the pressure/ flow setting everytime the flow changing

You measure the pressure at the location of interest (e.g. destination/the tank nozzle for example).

If the pressure at the nozzle into the tank needs to be say 250 kPa (because this is a typical atmospheric 80 ft tank), this then becomes the setpoint to the pressure control valve. The PID controller will open/close the valve as appropriate until the process value (the pressure reading at the inlet nozzle) matches your setpoint (the 250 kPa).

No operator intervention required.

How do they control the flow accurately from each station? I doubt they control and measured it accurately.

Actually, each producer sending flow into the main gathering pipeline system not only measure the flow, they measure it VERY accurately. Usually, there is a LACT unit Lease Automatic Custody Transfer unit) at the tie-in. Usually, when entering a gathering system, it is custody transfer. Most custody transfer flow meteres are PD meters (e.g. Brooks) and are very accurate.


Depending on how often these stations come on-line or drop-out, you may be cycling your VFD quite a bit. Also, depending on how accurate your destination pressure needs to be, a VFD may not do the job (not accurate enough, not fast enough, cycling, etc.)

 

[devaxrayz]

Ashereng, from what i know that the pressure at tank inlet nozzle will remain the same (as long the level is not change). Whatever pump discharge pressure or any valve opening will not affect to tank inlet nozzle pressure. That is static/hidrostatic pressure.

the pressure at some point at some distance from the nozzle however will vary depending on flowrate going to the tank... the dynamic pressure. Thats the principle of back pressure isn't it...

regards,

-Rayz-

 

[Ashereng]

Ashereng, from what i know that the pressure at tank inlet nozzle will remain the same (as long the level is not change). Whatever pump discharge pressure or any valve opening will not affect to tank inlet nozzle pressure. That is static/hidrostatic pressure.

Hydrostatic head is a constant pressure imposed on the system (again, ignoring some minor effects here). It does not mean that the pressure at the nozzle stays constant with level.

For example, you have a cup of water and a straw taped to the side of the glass 3 cm off the bottom. The pressure at the bottom of the straw is a certain pressure. If you blow hard into the straw, does the pressure at the bottom of the staw will increase.

Put another way:

If Q=100 m3/h, and total dp=3000 kPa. The pump discharge is 3250 kPa, then the pressure at the tank nozzle is 250 kPa.

All else being equal, if the pump discharge is now 4250 kPa, the pressure at the nozzle will be about 1250 kPa (ignoring some minor effects here).

Or, alternatively:

All else being equal, if the valve starts to close, the total dp in the line will increase, and the pressure at the nozzle and/or the flow rate to the tank will change.


I am not quite sure what you mean wrt the second paragraph.

Back pressure means controlling the pressure upstream of the back pressure control valve/regulator. If the flow changes, then the dp downstream of the regulator to the tank will change since the flow is now different.

 

[devaxrayz]

If Q=100 m3/h, and total dp=3000 kPa. The pump discharge is 3250 kPa, then the pressure at the tank nozzle is 250 kPa.

All else being equal, if the pump discharge is now 4250 kPa, the pressure at the nozzle will be about 1250 kPa (ignoring some minor effects here).

The only way you can get increased discharge pressure to 4250 kPa is that the fluid FLOWRATE IS INCREASE so the pressure drop also increase to 4000 kPa. In the end the nozzle pressure will still REMAIN 250 kPa.

Or it is also possible if your piping diameter is reduced (i.e. scale problem) so at the same flow rate the pressure drop is higher.

The basic centrifugal pump i learn is that pressure does NOT build by the pump, but the RESISTANCE does build the pressure.

When you develop SYSTEM CURVE or RESISTANCE CURVE for pump system, you always set the destination (tank nozzle) pressure at constant at any flow you got. The more destination pressure, more long the pipe, more high the flowrate, more bad your piping condition, more pump head required. The pump operating flow and pressure is than the intersection between pump curve and system curve. You will see that the pump is following the resistance, and those 250 kPa is one of them.


That is what i mean with back pressure in para 2

I am not quite sure what you mean wrt the second paragraph.

it is named backpressure because you calculate it from the destination point, back to the point you want to know e.g. pump discharge

I have no comment for your illustration but i think it was not relevant in this case

cheers,

-Rayz-