19 replies to this topic

[jatinder]

Dear all,

 

How can I calculate the maximum shutoff head of centrifugal pump?

 

I have a closed loop system.

 

pump flow rate = 950m3/hr

Differential head = 5.95 bar = 60.65 mlc

highest elevation point in the sytem is 50 meter

discharge pressure = 7.45 bar

 

I hope that I have submit the complete data. Let me know, if more is required?

 

Thank you very much,

 

Jatinder

 

 

 

[ankur2061]

Jatinder,

 

The shut-off head for a selected pump model can be read from the Capacity vs Head curve of that pump and is supplied by the pump manufacturer.

 

Norsok Standard P-001 provides an estimation for shut-off pressure as follows when pump manufacturer information is not available:

 

Shut-off Pressure = Suction Pressure at Relieving Conditions + 1.25 times the differential pressure at the normal flow

 

Hope this helps.

 

Regards,

Ankur

[jatinder]

Dear Ankur,

 

Thank you very much for your quick response. One thing that I am little bit confused with is the pump suction pressure in a closed loop system. For a normal system, the concept about pump suction and pump discharge is all clear. At the same time, when it comes to closed loop system, pump suction and discharge becomes tricky.

 

As, mentioned by you in above formula for shut in pressure, what is the right way to chose pump suction in closed loop?

 

just like your other blog entries, it would be great if you write a new blog regarding pump suction and discharge in a closed loop ssytem!

 

Thank you very much!

 

Looking forward to it.

 

Jatinder

[ankur2061]

Jatinder,

 

What definition or understanding do you have in mind for a closed-loop system? I need to understand your understanding of this before I attempt to provide you an answer.

 

Regards,

Ankur

[fallah]

jatinder,

 

In a closed loop system, the suction pressure would almost be the pressure corresponding to the static height of the highest point of the relevant piping system..

[jatinder]

Ankur,

According to my understanding:

 

A closed system is that where the fluid keeps on revoling in the same line. To add more into it, I can say that the pump suction is equal to pump dischage as it is difficult to separate the suction line from discharge line.

 

The pump head would be equal to corresponding head of friction loss in whole loop.

 

Jatinder

[jatinder]

Dear Fallah,

 

Thanks for your response. I appriciate it.

 

But what about discharge pressure than?

 

Jatinder

[fallah]

 

But what about discharge pressure than?

 

jatinder,

 

Discharge pressure would be the pressure corresponding to the static height of the highest point of the relevant piping system plus the frictional loss in whole piping loop...

[ankur2061]

Jatinder,

 

A typical closed loop system would be a heat transfer fluid or hot oil circulation loop connected to an expansion tank wherein the expansion tank is providing the suction for the pump and the discharge circuit is also connected to the expansion tank after passing through the consumers. Refer the attached sketch for a closed loop pumping system for a heat transfer fluid.

 

If you refer the sketch, the static head for the suction and discharge circuits is quite similar but not the friction or head losses. The head losses on the discharge circuit are much higher than the suction side which is directly connected to the pump suction.

 

Regards,

Ankur

Attached Files

•  Closed_Loop_Pumping_System.pdf   23.76KB   74 downloads

[jatinder]

Ankur,

 

Can a closed loop system still be called as closed loop if there is no expansion vessel present?

 

Jatinder

[ankur2061]

Jatinder,

 

Expansion vessel is just a name for a vessel in a hot oil system which serves to account for the thermal expansion of hot oil when the hot hoil expands due to rise in temperature.

 

A closed loop system requires that the pump circulates in a closed loop from suction to discharge, whether an expansion vessel is there or not. The example of a hot oil system was provided because it is one of the most common closed-loop pumping systems encountered.

 

Regards,

Ankur.

[Soothsayer]

Hi,

 

You need the pump curve to estimate the shutoff pressure. You could review some pump catalogues for that. I always use an estimation when I don't have the curve information; Maximum suction pressure plus a 120 % of the differential head of the pump.

[Purnesh Meshram]

Hi Jatinder

 

I use this formula irrespective of case;

SOH= Suction Vessel PSV set pressure (if PSV not present consider Vessel design Pressure) + Suction Static head                     above pump center line+Pump Differential Head- Pump discharge static head above pump center line.

 

this gives you Shutoff head until Pump vendor curves are available.

hope this will help.

Edited by chemtechiee, 20 November 2013 - 08:57 PM.

[Purnesh Meshram]

More specifically 

The shut-off pressure, SOP, is determined by the equation:

SOP = SP + HPS + HPO - HPD,

 

in which

- SP = set pressure of the pressure relief device on the pump suction system 

- HPS = hydrostatic pressure of the liquid above the pump suction

- HPO = pump differential pressure at no flow and maximum pump speed and highest density as per process design 

- HPD = hydrostatic pressure of the liquid above the pump discharge 

[jatinder]

Dear all,

 

I would like to thank you all for your posts. It is quite clear to me.

 

Ankur & Fallah, thanks a lot for your efforts.

 

Kind regards,

 

Jatinder

[S.AHMAD]

Closed-loop system MUST have expansion tank otherwiae the piping will be subjected to thermal expansion. Energy input to the pump will cause the fluid temperature to rise and leads to high pressure due to thermal expansion. The least to happen is leaking at the flanges. The worst thing is to have pipe or pump casing rupture.... think about it...

[fallah]

Closed-loop system MUST have expansion tank otherwiae the piping will be subjected to thermal expansion.

 

AHMAD,

 

Closed loop pumping system without an expansion tank is similar to a pump minimum flow line back to the suction line during discharge blocked outlet. It can be without an expansion tank if has been passed through a cooler before connecting to the suction line...

[S.AHMAD]

Fallah

1. Thanks for your points

2. But pump minimum line is normally return back to the source from  a tank or pressure vessel. Therefore it is NOT a closed system.

3. We DO NOT recommend a pump minimum  return back to the pump suction downstream of any source (e.g. tank) isolating valve. For simple reason that a closed-loop system can occur when the isolating valve and the pump discharge are closed simultenously(let say during start-up the operator inadvertently close the two valves and suddenly start the pump... this is also the reason an operator should open the pump suction valve first before starting a pump). Under this situation, thermal expansion will occur. Therefore the best configuration is to send the minuimum return line back to the tank or upstream the isolation valve. With this piping configuration, there is no closed-loop.

4. I hope above short explanation is clear enough or someone need to draw the piping configuration and study the drawing carefully.

5. Please give me another scenario of a closed-loop system without an expansion tank for further discussion. Of course, for pump return line scenario, alternative to expansion tank is to install TRV to protect the system from overpressure.

Edited by S.AHMAD, 21 November 2013 - 09:17 PM.

[fallah]

 

2. But pump minimum line is normally return back to the source from  a tank or pressure vessel. Therefore it is NOT a closed system.

3. We DO NOT recommend a pump minimum  return back to the pump suction downstream of any source (e.g. tank) isolating valve.

 

AHMAD,

 

You didn't get my point. You mentioned that a closed loop system must have expansion tank and i wanted to show, by a similarity: closed loop is similar to a minimum flow line back to the pump suction, under some conditions it can have not. Means i didn't recommend minimum flow back to the pump suction and even i did already recommend never using such configuration. Please see 7th post in following link:  

 

 http://www.cheresour...+fallah+suction

[S.AHMAD]

Fallah

1. I am referring to a BIG system. Pump recirculation line as shown on attached drawing is small and TRV is normally sufficient to cater for the situation.

2. In the attached drawing, NEVER install the minimum recirculation line using this configuration. If the pump discharge valve is closed, the system will be subjected to thermal expansion. This is an example of closed-loop and need a protectection from overpressure. For this small system, TRV is sufficient.

3. However, If the suction line check valves are removed then the system is a closed-loop with source tank as the expansion tank.. The check valve at the suction line is NOT necessary. I believe the engineer proposed this system is INEXPERIENCE or he has a good teason that I do not know. Let me know please.

4. If the check valves are required for some reasons, the minimum return line should be upstream of the ceck valves so that the source tank becomes the expansion tank.

Attached Files

•  Pump with a Recirculation Loop.pdf   41.76KB   28 downloads

Edited by S.AHMAD, 22 November 2013 - 06:07 AM.