level switch vs pressure switch - PLCS.net

Charbel · July 25th, 2014, 04:45 AM

Dear,

For protecting the pump against dry running, it is better to use low level switch in the tank or pressure switch on the suction side?

I tend to go for low level switch in the tank to protect the pump from dry run, please advise of any thought.

thank you!

charbel

nonuke · July 25th, 2014, 05:49 AM

Why not use a flow switch after the pump of the type: IFM - SI5000

John Soltesz · July 25th, 2014, 08:59 AM

Another way is to monitor the pump inlet line with a point sensor switch. If there is liquid then OK to pump.
Look at the E +H Liquiphant FTL20 types of sensors.

Mickey · July 25th, 2014, 09:07 AM

I don't think one can say which is better. But I would go with the level switch.

Most tanks will have a low and high for control and a Low-low and High-High for protection. The extra switches provide extra protection if the low does not shut the pump off the low-low will.

Then again a pressure switch closed to the pump would protect against closed or pinched valves.

Your installation will dictate. Maybe you need both.

Tom Jenkins · July 25th, 2014, 09:10 AM

For protection against running dry the level switch, regardless of type, is probably the better bet. A low pressure (high suction) switch on the suction side of the pump would be used to protect against cavitation. If you have a flooded suction pump with normal piping that is probably not a problem.

TConnolly · July 25th, 2014, 10:21 AM

Level switch is the simplest.

There is however another option to consider if level switch installation is challenging. www.loadcontrols.com has a sensor where you place a CT around a motor lead wire and it can detect pump dry run. It also works if someone closes the pump suction valve.

nonuke · July 25th, 2014, 10:59 AM

If you use a level switch, you probably also need a pressure switch because you might have a closed valve after the pump (liquid but no flow). If you use a flow switch you don´t need any other switch.

Charbel · December 12th, 2014, 05:07 AM

Dear,

I assume a flow switch will needs to be located after the pump, because a flap type flow switch before the pump will not make a difference but on the contrary it will stop the pump from running because the pump will need to run to make flow and to make the flow switch work.
I might use a thermal based switch on the discharge but am i not being too late to stop the pump?, the thermal based switch will have to detect no flow in order to stop the pump which is too late?

I think the best would be pressure switch on suction combined with low level switch on the upstream tank

Any thought about this?

Thank you!

charbel

realolman · December 12th, 2014, 08:39 AM

I don't think I'd bother with the pressure switch at all

I think it would be more trouble than it would be worth

The flow switch can be a pain as well, and you will have to ignore it till you get the pump started

use floats

Rumiat · December 12th, 2014, 09:38 AM

Well it all depends on your system actually.

If you have a foot valve installed (assuming tank is lower than pump) that would make sure your pump remains primed, just go for a low level switch.

If you choose to use a flow switch, I think you may have to bypass the flow safety system during pump starting and adjusting its suction/discharge valves. So low level is a good option I would say.

Hope this helps.

nonuke · December 12th, 2014, 10:19 AM

I also use a level limit switch model Liquiphant FTL20 (vibrating forks type) before the pump. If pipe empty, the pump won't run. Works all the time.

July 25th, 2014, 04:45 AM	#1
Charbel Member Join Date: Jan 2012 Location: Beirut Posts: 307	level switch vs pressure switch Dear, For protecting the pump against dry running, it is better to use low level switch in the tank or pressure switch on the suction side? I tend to go for low level switch in the tank to protect the pump from dry run, please advise of any thought. thank you! charbel

July 25th, 2014, 09:07 AM	#4
Mickey Lifetime Supporting Member Join Date: May 2003 Location: Palmdale,Ca Posts: 10,611	I don't think one can say which is better. But I would go with the level switch. Most tanks will have a low and high for control and a Low-low and High-High for protection. The extra switches provide extra protection if the low does not shut the pump off the low-low will. Then again a pressure switch closed to the pump would protect against closed or pinched valves. Your installation will dictate. Maybe you need both. __________________ Mickey If you want happiness for an hour-take a nap. If you want happiness for a day-go fishing. If you want happiness for a month-getmarried. If you want happiness for a year-inherit a fortune. If you want happiness for a lifetime-help someone else. ----- Chinese Proverb

July 25th, 2014, 10:21 AM	#6
TConnolly Lifetime Supporting Member Join Date: Apr 2005 Location: Salt Lake City Posts: 6,152	Level switch is the simplest. There is however another option to consider if level switch installation is challenging. www.loadcontrols.com has a sensor where you place a CT around a motor lead wire and it can detect pump dry run. It also works if someone closes the pump suction valve. __________________ True craftsmanship is only one more power tool away. That's the beauty of processors, they don't have emotions they just run code - The PLC Kid.

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July 25th, 2014, 05:49 AM	#2
nonuke Member Join Date: Sep 2005 Location: Posts: 840	Why not use a flow switch after the pump of the type: IFM - SI5000

July 25th, 2014, 08:59 AM	#3
John Soltesz Member Join Date: May 2007 Location: Pemberville, Ohio Posts: 260	Another way is to monitor the pump inlet line with a point sensor switch. If there is liquid then OK to pump. Look at the E +H Liquiphant FTL20 types of sensors.

July 25th, 2014, 09:10 AM	#5
Tom Jenkins Lifetime Supporting Member Join Date: Apr 2002 Location: Milwaukee, WI Posts: 6,270	For protection against running dry the level switch, regardless of type, is probably the better bet. A low pressure (high suction) switch on the suction side of the pump would be used to protect against cavitation. If you have a flooded suction pump with normal piping that is probably not a problem.

July 25th, 2014, 10:59 AM	#7
nonuke Member Join Date: Sep 2005 Location: Posts: 840	If you use a level switch, you probably also need a pressure switch because you might have a closed valve after the pump (liquid but no flow). If you use a flow switch you don´t need any other switch.

December 12th, 2014, 05:07 AM	#8
Charbel Member Join Date: Jan 2012 Location: Beirut Posts: 307	Dear, I assume a flow switch will needs to be located after the pump, because a flap type flow switch before the pump will not make a difference but on the contrary it will stop the pump from running because the pump will need to run to make flow and to make the flow switch work. I might use a thermal based switch on the discharge but am i not being too late to stop the pump?, the thermal based switch will have to detect no flow in order to stop the pump which is too late? I think the best would be pressure switch on suction combined with low level switch on the upstream tank Any thought about this? Thank you! charbel

December 12th, 2014, 08:39 AM	#9
realolman Member Join Date: Mar 2009 Location: here Posts: 584	I don't think I'd bother with the pressure switch at all I think it would be more trouble than it would be worth The flow switch can be a pain as well, and you will have to ignore it till you get the pump started use floats

December 12th, 2014, 09:38 AM	#10
Rumiat Member Join Date: Dec 2014 Location: Lah Posts: 2	Well it all depends on your system actually. If you have a foot valve installed (assuming tank is lower than pump) that would make sure your pump remains primed, just go for a low level switch. If you choose to use a flow switch, I think you may have to bypass the flow safety system during pump starting and adjusting its suction/discharge valves. So low level is a good option I would say. Hope this helps.

December 12th, 2014, 10:19 AM	#11
nonuke Member Join Date: Sep 2005 Location: Posts: 840	I also use a level limit switch model Liquiphant FTL20 (vibrating forks type) before the pump. If pipe empty, the pump won't run. Works all the time.

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