10 replies to this topic

[B.M. Process]

Hi,

 

We have a condition where a contractor installed two identical pumps with common suction header and different suction pipes; one 6" and the other 8''. This is contrary to the detailed design stipulations (both were supposed to be each 8"). When the pumps were commissioned, it was difficult to achieve a balance and thus it was difficult to operate the two pump simultaneously as per the intended operation philosophy. 

 

Personally, I think it is a bit of a problem because the losses will be higher in the one with small size diameter.  Please if you have any more problems that you think could be affecting the pumps with different suction pipes, it will be appreciated. Oh, the suction vessel is a tank which is always full and we do not expect any case of low flow. Remember too, that the pumps with smaller diameter suction pipe is closer to the suction tank (See the rough sketch).

 

 

This a strange arrangement for i have never seen it. I guess it is true for two dissimilar pumps but not for identical pumps.

 

Your input is appreciated!

 

Attached Files

•  parallel.docx   29.89KB   30 downloads

[fallah]

Hi,

 

Please specify if there is common discharge header for these two pumps...or as the sketch shows there is not...

[B.M. Process]

There is a common discharge header (see the ketch).

 

Thanks!

Attached Files

•  parallel.docx   33.71KB   23 downloads

[samayaraj]

Hi Bmthonshah,

 

You need to provide us flow details, suction head, length of suction piping, layout etc for evaluation.

 

As per your sketch, in 10 inch 40 sch pipe and allowed velocity of 1 m/sec, the flow is approx 182 m³/hr. So, each pump should deliver 91 m³/hr flow if they are running simultaneously. So in 6 inch 40 Sch pipe, for 91 m³/hr flow, the velocity found to be nearly 1.35 m/sec. This will have impact on pump NPSH. But in actual it may not happen. Because the flow will try to go in least resistance path and both the pump will have different flow because of different in suction pipe size. This will have impact on discharge and pressure at the pump outlet will be different.

 

Also, when pumps are operating in parallel, the average of cumulative flow is less than the pump's flow if it is operated individually.

Edited by samayaraj, 21 April 2016 - 06:37 AM.

[B.M. Process]

Dear Samaraj,

 

Thanks for your response,

 

As for the information required to fully analyse the problem, I have the following info:

1. pump differential pressure (12MPa)  -> (12,000,000/(974.3*9.81)=1255.4m

2. common discharge header is 10", sch.20, and 25.

3. flow rate is 80 cubic meter per hour

3. Find the attachment for more info.

 

It is true that the accumulative or total flow (Qt) will  always be less twice that of a single pump and the reason is because the friction resistance increases when both pumps run simultaneously. The head on the other hand, will be the same as that of one pump. So, if we run both pumps as we intend to do, we should expect less than (80*2 =160 cubic meter per hour).

 

Sorry, I could not get the isometric drawings and/or the pipe alignment sheet.

I hope the information I provide will give you more insights in visualizing the problems.

 

Rgds,

Attached Files

•  info.docx   837.78KB   13 downloads

Edited by Bmthonshah, 21 April 2016 - 07:46 AM.

[Mahdi1980]

Hi,

As far as I know , What affects the unbalance operation in parallel similar pumps is discharge pipe diameter since, it impacts on the friction loss and leading to pressure drop of the system. So, different size discharge pipes would affect the different system curves and may cause a problem for your pumps.

Different suction lines with different diameters may just affect the NPSHa of pumps.

For instance in your case, line 6" has a higher pressure drop than the 8". So, may take the pump nearer to the vapor pressure of fluid.

And if the pump is not well selected may cause a problem like cavitation for that one installed on 6".

So as brief:

1- if the size of discharge lines are well selected so that can handle the required flow rate there is no a dramatic problem.

2- different diameter suction lines to be checked to avoid cavitation in pumps.

3- Minimum flow lines to be connected

4- it is advised to use identical discharge lines to have a better operation in simultaneous services.

5- Take note that Two simultaneous pump operation will deliver the lower total capacity than sum of each single pump flow .

 

If you have another question please let us know.

 

Mahdi

Edited by Mahdi1980, 21 April 2016 - 07:54 AM.

[B.M. Process]

Thanks, Mahdi, for your input.

 

As I said right for the start of this conversation, the losses in the 6'' size will be higher and will subsequently impact the NPSHa against the NPSHr which is 5.5m according to the vendor document. My plan thus far, is to recalculate the NPSHa  for the pumps with 6" size to evaluate as to whether or not it will satisfy the requirement that NPSHa > NPSHr +1m. The fluid we are dealing with here, is produced water (pw) with the properties shown on the attachment in my recent response.

 

 

Our biggest problem we are facing where I work is procurement; it takes months and months to get the material on site. This therefore, forces the contractor to cut corners by not following the design. So far, we have issued non-compliance notice (NCR) to the contractor and we will see if NPSHa is not feasible, we will dismantle the piping and install 8'' size. 

 

Thank you so much for your contribution!

[harshad katre]

Dear Sir,

 

I was going through this discussion and found "Geodetic" word. Will you please elaborate what this menas?

 

Also I want to add into this that if pump is at positive suction then how NPSH will affect the performance of pump?

 

Thanks in advance.

 

Regards,

Harshad Katre

Edited by harshad katre, 22 April 2016 - 12:19 AM.

[shantanuk100]

Hello Harshad,

 

1. The Geodetic head is nothing but head on account of height above a geodetic Datum (Datum is a reference height line).

 

2. The Geodetic head is defined as the actual physical difference in height between the liquid level in the pit/vessel from where it is being pumped, to the highest point of the discharge of the same fluid.

 

3. In simple terms, it is the physical height difference between the liquid level surface and its discharge point level.

 

4. This height can change when the pump is running due to water level fluctuations, which is why the geodetic head is defined as the average water level between start and stop conditions, while sizing a pump.

 

Regards,

Shantanu

[harshad katre]

Thanks Shantanu!!

[S.AHMAD]

Hi there

 

After studying the diagram, I think no operational problem since each pump has it own flow control valve. Even  we can operate smoothly with only 1 common valve for both pump. I can explain it in more detail if you attach the pump curve.

 

Salam from S.Ahmad