10 replies to this topic

[ReddeerC]

Dear Colleagues,

 

My thoughts are battling to see if a PSV has to be added to protect a booster pump with a recirculation line as shown in the attached sketch.

 

The booster pump is far from the pumped fluid source and to be working in a remote area. Client doesn't want to install a tank for the pump; therefore, the pump recirculation line has to be routed back directly to the pump suction line.

 

For some operation scenarioes, the pump discharge flow Q_discharge is close to the recirculation flow Q_{recirculation} (the foward flow Q_forward is small), and the pump discharge pressure under such scenarioes are close to the pump shutoff head. What my concern is: if the flow control valve on the recirculation line accidentally widely open under such scenarioes, the pump suction line will see the pump discharge pressure (shutoff pressure). Then, this upset suction pressure will be added by the pump TDH repeatedly (because of the flow recirculation) and soon the pump dicharge pressure might reach to the deisgn pressure of the pump casing and pipeline.

 

Such concern tempts me to add a PSV to the pump discharge line to protect the pump and pipeline. Do you think my concern is valid and it is necessary to add a PSV?

 

 

Thanks for your feedbacks in advance.

Attached Files

•  Pump with a Recirculation Loop.pdf   41.76KB   177 downloads

Edited by ReddeerC, 16 November 2013 - 09:05 PM.

[fallah]

For some operation scenarioes, the pump discharge flow Q_discharge is close to the recirculation flow Q_{recirculation} (the foward flow Q_forward is small), and the pump discharge pressure under such scenarioes are close to the pump shutoff head. What my concern is: if the flow control valve on the recirculation line accidentally widely open under such scenarioes, the pump suction line will see the pump discharge pressure (shutoff pressure).

 

ReddeerC,

 

Please clarify under the scenario in which Q_discharge is close to the recirculation flow Q_{recirculation} , if the flow control valve on the recirculation line accidentally wide opened, how the pump discharge pressure is going to be at shut off?

[Santoshp9]

ReddeerC,

If CV on RC line opened wide then pressure drop across CV will be reduced & ultimately the system resistance will reduce,so the pump discharge pressure is not going to increase. But if the discharge flow is very very less then due to recirculation itself within casing causes back pressure & it will show as Shut off head.

 

Regards,

Santosh

[Zauberberg]

Red Deer,

 

The pump can never exceed its shut-off pressure, which occurs at zero flow through the pump. Thus having the spillback valve open poses no concerns with regards to this subject - you can never exceed TDH developed by the pump, at any flow.

[ReddeerC]

Thanks, fallah, Santosh and Zauberberg for your time and inputs.

 

I provide more information here. The booster pump minimum continuous stable flow is 35 m3/h and at this flow rate pump TDH is 250 m which is very close to the pump shutoff head (253 m) according to pump curve. Because the pump suction line is from a low pressure system (150#) and the booster pump belongs to a higher pressure system (300 #), two check valves and a PSV are installed on the pump suction line and before the location where the recirculation line meets the suction line. To prevent confusion, let's assume an extreme situation: the booster pump downstream is blocked and there is no forward flow, only recirculation flow from the pump discharge back to the pump suction line. Let's forget the temperature issue, but just focus on the pressure side. Please refer to revised sketch.

 

I know that the pump TDH can never exceed its shut-off pressure. My concerned point is:  when the pump only sees recirculation flow and if the control valve widely open, then the pump developed TDH (250 m) will be  transferred to the suction line by the recirculation flow. Thus, the suction line pressure will be increased.  Since the pump is running and continously adding energey (developed TDH) to the flow,  then the pump both discharge and suction pressures would be increased continuously (because of the repeatedly superimposition of the developed TDH on the recirculation flow from the pump discharge back to the pump suction). In the end, the pump might be overpressured. This is what my concern is. But I am not sure if this concern is valid.

Attached Files

•  Pump with a Recirculation Loop.pdf   41.76KB   98 downloads

Edited by ReddeerC, 16 November 2013 - 09:50 PM.

[Zauberberg]

Continuous, prolonged circulation will result in continuous temperature increase of the circulating fluid. As discussed in another topic, you need to calculate for how long you can operate in 100% recycle mode before you start damaging the pump. Shutdown of the pump should be in place, once you approach the maximum suction temperature. Pump datasheet should contain this information.

 

Regarding your other concern - pressure increase -  I don't see it as a possible scenario, particularly if pressure in the pump suction vessel is controlled via PIC loop. Loss of forward flow will increase liquid level in the suction vessel (I presume there is one upstream of the pump), and the whole system will trip on high-high level, so there will be no accummulation of mass inside the system.

 

You didn't mention which safety functions (interlocks, alarms, and trips) exist in the pumping system.

[fallah]

ReddeerC,

 

In fact, minimum flow recirculation line should never be routed back to the pump suction line. It will cause overheating of the recirculated flow and abnormal turbulency in pump suction line. It can be applied for short-term, let say few minutes, and if routing back to the suction vessel would be impossible, the recirculation line for long-term should be included an inline cooler before connecting to the suction line.

 

Indeed, the reported minimum flow (35 m3/hr) might be specified by vendor for normal situation with recirculation line back to the suction vessel and in new situation it might not be passed through flow control valve. Finally, in new situation due to nonstable operation the pump might not follow its performance curve entirely...

[ReddeerC]

Thanks again, fallah and Zauberberg, for your inputs.

 

As mentioned at the beginning of the post, this is a booster pump in remote area which is far from the supply tank and pump. Therefore, the recirculation line is routed back to the booster pump suction line.

 

Yes, the temperature rise during recirculation, especially when the forward flow is also very small,  is a serious problem and this issue has been considered already: a high temperature alarm and temperature switch are to protect the pump. Or, as fallah suggested, I could include an inline cooler, or dictate the forward flow always larger than the minimum continuous thermal flow to resolve the problem.

 

However, what I really want to know is if such recirculation line design will cause overpressure of the pump and pipeline during the upset situations (e.g., blocked downstream or forward flow is very small compared to the recirculation flow). This is also the reason why I said in the last reply " Let's forget the temperature issue, but just focus on the pressure side."

 

Most possibly the maximum discharge pressure of the booster pump at any conditions would be "Max supply pump suction line pressure+ supply pump shutoff head + the booster pump shutoff head". But I am not 100% sure with such recirculation line design for the booster pump.

 

Thanks again anyway.

Edited by ReddeerC, 17 November 2013 - 09:20 PM.

[Zauberberg]

Without knowing how the system upstream of the pump would behave in case of blocked discharge flow, it is impossible to answer you query. Look at my previous post and see if you can provide us with an answer.

[reena]

Reddeer, No pump, if running and having a forward flow will pressurize its suction. But there is another scenerio, where the suction pressure can go up substantially, even damaging pipes, flanges, etc. That is when the booster pump trips with the recirculation valve open (even partially). This is because the pump downstream pressure is higher ( otherwise the pump would not be required) and it can flow into the suction, provided the Tap-off for recirculation is taken downstream of pump NRV(normal case). This is like the settle-out pressure situation in a multi-stage centrifugal compressor circuit. The suction side will experience higher pressure due to baack flow. This is the case you have to consider rather than the one you are talking about.

[ReddeerC]

Thank you very much, everyone, for your time and kind help with this topic.

 

I am for Reena's comments.