13 replies to this topic

[Ariel]

Hi!! Can anybody help about these two situations?

A) Several times I have seen centrifugal pumps protected by pressure transmitter mounted on suction side. The idea of these instruments is to indicate no flow conditions sensing pressure. But that works? The alarm set is usually atmospheric pressure (when liquid level is loss) but…..

1) What happen when I have vacuum at the pump suction under normal operation? It seems that this kind of protection is useless.
2) What happen if, by mistake, an operator close pump suction valve? The centrifugal pump makes low pressure or vacuum enough to be detected by the transmitter?

Based on this I prefer Flow Switch for pump protection.


On the other hand, I have seen pump protected by pressure transmitter mounted on the discharge. I am not sure about that kind of protection is good for small pumps, where the difference between normal operation point pressure and shut off pressure is (let say) only 0.5 Kg/cm2g.
For example, think about a tank and pump system working under two different condition. . At LLL operation condition the pump is taking water from a tank when the level is 2 meters, the discharge pressure is (let say) 2 Kg/cm2g. Follow this we would set the alarm to stop the pump at 2.5 Kg/cm2g (0.5 Kg/cm2g more than the normal pressure: shut off pressure).
But, at HLL operation condition the pump is taking water from a tank when the level is 8 meters, the discharge pump pressure will be 2.6 Kg/cm2g (increased by the head of water) and, under the previous scheme’s of pump protections, the pump would stop immediately.

Based on this I think that mounting pressure transmitter on the discharge doesn’t work for small pump. You need a good difference between normal operation point pressure and shut off pressure. So for this case a prefer Flow Switch for pump protection.


About points A and B. What do you think? Do you agree? I am missing something. I would appreciate very much your opinion.
Thanks

Ariel

[JoeWong]

.....Several times I have seen centrifugal pumps protected by pressure transmitter mounted on suction side. The idea of these instruments is to indicate no flow conditions sensing pressure. But that works? The alarm set is usually atmospheric pressure (when liquid level is loss) but…..

1) What happen when I have vacuum at the pump suction under normal operation? It seems that this kind of protection is useless.
2) What happen if, by mistake, an operator close pump suction valve? The centrifugal pump makes low pressure or vacuum enough to be detected by the transmitter?

Based on this I prefer Flow Switch for pump protection.

If NPSHa lower than NPSHr, then pump will cavitate.
Pressure transmitter at pump suction may be used to avoid pump operate under cavitation.


With pump suction valve closed, internal recirculation may lead to cavitation & pump damage. Doubt centrifugal pump can create vacuum.

.....On the other hand, I have seen pump protected by pressure transmitter mounted on the discharge. I am not sure about that kind of protection is good for small pumps...

Pressure transmitter on pump discharge is normally used to trip pump on high discharge pressure and avoid opening of discharge PSV.

[pawan]

Surprised that U will consider such a pump which gives a difference in Shutoff & Normal pressure less than your tank level difference HLL & LLL. I wont consider that kind of design in the first place. Even if Its a requirement due to other constraints I will not protect it based on shut off pressure. In this consition I will put a trip interlock at HLL & LLL with a consideration of lower HLL. Say may be 6 meters in place of 8 meters.

I dont find any better way than tank level monitoring for controlling pump safety except some special situations.

Anyway I wonder what will U do once ur tank level HLL goes to 8 meters. In that case U cant operate ur pump. So how do U reduce it again to normal value without pumping.

Also I dont see any vacumm transmitter in the suction bcoz in case of positive pressure systems U will never find vacuum at any point in suction lines unless boiling liquids are there.

On flow meter based system I am again negative as flow sensing would come much later than level sensing by the time ur impellers might get damaged in case of low shutoff pumps.

[Ariel]

.....if NPSHa lower than NPSHr, then pump will cavitate.
Pressure transmitter at pump suction may be used to avoid pump operate under cavitation.
With pump suction valve closed, internal recirculation may lead to cavitation & pump damage. Doubt centrifugal pump can create vacuum.

Dear Joe,

With “If NPSHa lower than NPSHr, then pump will cavitate” I totally agree. But with “Pressure transmitter at pump suction may be used to avoid pump operate under cavitation” how can I get it? I chose a set alarm next to the boiling point of the fluid…but it isn’t vacuum for fluid like water or crude oil taking from an open tank?

thanks very much for you time

.....On the other hand, I have seen pump protected by pressure transmitter mounted on the discharge. I am not sure about that kind of protection is good for small pumps...

Pressure transmitter on pump discharge is normally used to trip pump on high discharge pressure and avoid opening of discharge PSV.

OK, I Agree.

[Ariel]

Surprised that U will consider such a pump which gives a difference in Shutoff & Normal pressure less than your tank level difference HLL & LLL. I wont consider that kind of design in the first place. Even if Its a requirement due to other constraints I will not protect it based on shut off pressure. In this consition I will put a trip interlock at HLL & LLL with a consideration of lower HLL. Say may be 6 meters in place of 8 meters.

Ok this solution is useful with an existing installation. Anyway you encourage my opinion that this kind of installation in not well designed. I want to remark that the installation I gave is just an example, is not real.

I dont find any better way than tank level monitoring for controlling pump safety except some special situations.
Anyway I wonder what will U do once ur tank level HLL goes to 8 meters. In that case U cant operate ur pump. So how do U reduce it again to normal value without pumping.

Usually the pump operator would operate the pump with out the high pressure protection. But like i said, it isn’t a real installation, just an example.

Also I dont see any vacumm transmitter in the suction bcoz in case of positive pressure systems U will never find vacuum at any point in suction lines unless boiling liquids are there.

On flow meter based system I am again negative as flow sensing would come much later than level sensing by the time ur impellers might get damaged in case of low shutoff pumps.

I don’t agree with that. In my opinion you could have small vacuum at pump suction (probably you will have a particle filter at the suction loosing pressure). Because of that, a pressure transmitter at the suction side is completely useless.
About flow meter protection, yes it’s very possible that by the time the no flow condition is detected the pump would be running under cavitation condition for a while.

Thank you very very much for your recommendations.

[mishra.anand72@gmail.com]

I have seen pump protection on low tank level and/or low flow but I have never seen on low suction pressure. Discharge pressure high protection do exist in case pump is discharging into system that had been designed at lower pressure than pump shut off prssure.

[Ariel]

I have seen pump protection on low tank level and/or low flow but I have never seen on low suction pressure. Discharge pressure high protection do exist in case pump is discharging into system that had been designed at lower pressure than pump shut off prssure.

mishra,

Thanks for your comment. Where I work is very common this kind of protection, but I still can’t accept it. Maybe because I don`t understand it and this is why I ask for another engineers recommendations.
The two kind of protection you are telling are the ones I prefer. We use Discharge pressure high protection to avoid out of BEP or shut off running conditions (protect the pump from fluid overheating, seals fail in the case the use the pumped fluid for cooling, vibrations, etc)

Thanks and Bye

Like many others, I have not seen pump protection from pressure transmitter on suction line. We typically use low-low interlock on tank level to shut down pump. Also use motor manager on pump that will shut down pump on low amp draw to protect against cavitation. Also, use high amp trip on motor manager to protect against over pressure (in conjunction with pressure switch or transmitter in discharge line). High amp cut-out also protects (somewhat) against damage if impellar or other internals start to rub.

[JoeWong]

With “If NPSHa lower than NPSHr, then pump will cavitate” I totally agree. But with “Pressure transmitter at pump suction may be used to avoid pump operate under cavitation” how can I get it? I chose a set alarm next to the boiling point of the fluid…but it isn’t vacuum for fluid like water or crude oil taking from an open tank?

This type application (avoid cavitation) typically for pumping liquid at saturation at the suction drum and having complicated suction line, insufficient static head, process system doesn't afford unstabil flow, etc.

Another reason i missed out is create full or partial vacuum at the pump suction would invites air ingress into the process system. (Read More click here)

[Ariel]

Like many others, I have not seen pump protection from pressure transmitter on suction line. We typically use low-low interlock on tank level to shut down pump. Also use motor manager on pump that will shut down pump on low amp draw to protect against cavitation. Also, use high amp trip on motor manager to protect against over pressure (in conjunction with pressure switch or transmitter in discharge line). High amp cut-out also protects (somewhat) against damage if impellar or other internals start to rub.

Thanks Merrill
It´s safe to use low-low interlock on tank level to shut down pump provided nobody close a suction valve. In that case others protection must act like high discharge pressure or hicg motor amp.
see you

[Ariel]

With “If NPSHa lower than NPSHr, then pump will cavitate” I totally agree. But with “Pressure transmitter at pump suction may be used to avoid pump operate under cavitation” how can I get it? I chose a set alarm next to the boiling point of the fluid…but it isn’t vacuum for fluid like water or crude oil taking from an open tank?

This type application (avoid cavitation) typically for pumping liquid at saturation at the suction drum and having complicated suction line, insufficient static head, process system doesn't afford unstabil flow, etc.

Another reason i missed out is create full or partial vacuum at the pump suction would invites air ingress into the process system.

Thanks Joe!!!!

To be closing this topic I conclude:

• Mount pressure transmitter at the suction is not a common method of pump protection. Beside that, it’s not one of the best.
• Protect your pump with: tank level monitoring, high discharge pressure and high motor amp.
• As long as you can avoid vaccum at suction side of the pump (avoid air ingress)
• Try to avoid the next design “not consider a pump which gives a difference in Shutoff & Normal pressure less than your tank level difference HLL & LLL”

Thanks to all!!!!!!!

[venkat1976]

Like many others, I have not seen pump protection from pressure transmitter on suction line. We typically use low-low interlock on tank level to shut down pump. Also use motor manager on pump that will shut down pump on low amp draw to protect against cavitation. Also, use high amp trip on motor manager to protect against over pressure (in conjunction with pressure switch or transmitter in discharge line). High amp cut-out also protects (somewhat) against damage if impellar or other internals start to rub.

Thanks Merrill
It´s safe to use low-low interlock on tank level to shut down pump provided nobody close a suction valve. In that case others protection must act like high discharge pressure or hicg motor amp.
see you

Please anyone send me a spreadsheet that can be utilized in pipeline designing

[JoeWong]

Thanks Joe!!!!

To be closing this topic I conclude:

• Mount pressure transmitter at the suction is not a common method of pump protection. Beside that, it’s not one of the best.
• Protect your pump with: tank level monitoring, high discharge pressure and high motor amp.
• As long as you can avoid vaccum at suction side of the pump (avoid air ingress)
• Try to avoid the next design “not consider a pump which gives a difference in Shutoff & Normal pressure less than your tank level difference HLL & LLL”

Thanks to all!!!!!!!

Ariel,
You may have above as your own conclusion...

Always keep in mind, every design has it own unique feature and suitability. Don't discard any possibility until you have properly engineered it.

Good luck.

[Ariel]

Thanks Joe!!!!

To be closing this topic I conclude:

• Mount pressure transmitter at the suction is not a common method of pump protection. Beside that, it’s not one of the best.
• Protect your pump with: tank level monitoring, high discharge pressure and high motor amp.
• As long as you can avoid vaccum at suction side of the pump (avoid air ingress)
• Try to avoid the next design “not consider a pump which gives a difference in Shutoff & Normal pressure less than your tank level difference HLL & LLL”

Thanks to all!!!!!!!

Ariel,
You may have above as your own conclusion...

Always keep in mind, every design has it own unique feature and suitability. Don't discard any possibility until you have properly engineered it.

Good luck.

Joe,

Thanks for the advice and yes, once I heard that there’s no "Copy-Paste" projects, only "Copy-Paste" process engineers.
Thanks again!!!!!