level switch vs pressure switch for dry run protection

Dear,

I have a case where a centrifugal pump is sucking from a double compartment tank, usually for the dry run protection of the pump, we use low level switch.

in this case, we have to use two level switches for each compartment and the logic will get complicated if we close one compartment.

my question is what about using a pressure switch directly before the pumps, on the discharge of the tanks (their water height is around 2 meter), I really appreciate any feedback on the drawbacks or advantages in using a pressure switch on the suction line.

thank you!

charbel

Why not put a extra levelswitch in, one in the suction pipe of the pump?

Why not use a flow switch after the pump. Something like the SI5000 from IFM.

Agree with that, if you can, use a flow switch. put a simple logic like: if you start the pump and you have no flow after 3 sec , the pump stop and a alarm light go on .. and you need a manual button to reset it.

if you go for level switch, you will need to do the same add a timer and after X second if the switch still never activate, the pump go in alarm.

Pressure switch for dry run protection

dear,

thank you for your reply, but what about having a pressure switch on the suction of the pumps for the protecting the pumps against dry run, will it work properly, or it will cause problems?

thank you!

charbel

Or put the pressure switch on the pump discharge.

Depending on your installation, the normal running pressure at the pump suction could be lower than atmospheric. Therefore when the line is empty, i.e. open to atmosphere, the pressure looks good. If the pump NPSH requirement is not met, the discharge pressure will fall.

You would need a pretty sensitive pressure switch-possible IFM PN5. You don't have a lot of suction pressure to begin with 2 meters= 2.83psi and I don't know how much your level varies. I'd also think you'd want it a small distance away from the pump impeller (lower pressure). You might be better off with a discharge pressure switch (unless your using a VFD), flow switch or even motor current switch.

Keyance has small flow sw

Keyance has a small flow meter that straps around the pipe (1/2"- 2" I think) it can be set for any flow rate alarm.

We have had nothing but trouble using pressure switches on the suction side of a pump attached to a tank. I don't recommend it. Use a load monitor on the motor or output pressure switch.

Flow switch is the best option. Your pump will still maintain pressure with a shut discharge valve (but will start heating up quickly with no flow) . I know you're concerned with a low liquid level on the suction but why not monitor for both conditions.

Also if your pump loses prime and becomes airlocked, the suction side pressure transmitter would not pick this up. But a flow sensor would.

Endress do some good insertion type thermal loss switches (Flowphant I think is the model name). Works well for water anyway.

What is the purpose of dry ran switch?
Is it protect that pump ever goes to dry run mode and broken for that?
Can pump pump again without manually seed (what ever f.e. water) filling?
I think, separate dry run level switch is correct practice.

Use a flow switch on the pump output - they work. I often use them in conjunction with a current monitor - if the current falls to low it turns on and warns you. Before the pumps would be suction anyway and not reliable.

You could put a flow switch after the pump, as suggested above, then if your logic shuts the pump off due to no flow, instead of a manual reset, have a pressure switch on the suction pipe that is only monitored when the pump is NOT running. Have that the pressure switch must be on in your logic to start the pump & also restart the pump automatically when shut down. If you monitor the pressure switch while the pump is running it could be measuring a vacuum if the pump is pulling enough.

I did it few time also, if for some reason your company can't afford to buy a flow switch AND a pressure switch like mine

You can put just a flow switch and for the start use a timer, if after a start you dont have any low the pump go into fault like a noflow.

Based on past posts I am going to assume that this is a wastewater pumping station. Based on standard practice I am also going to assume that this is a flooded suction dry well pump, not submersible. In other words, the wet well level is higher than the pump suction connection.

If this is a sewage application do not put a standard flow switch on the line. Most of them have something projecting into the flow stream, and this will catch rags, disposable wipes, and other debris. This will either clog the line or damage the switch, and probably do both.

In a divided wet well it is common to have a suction bell for individual pumps and have multiple pumps, each drawing from only one wet well compartment. Then you have no problem. If the well that the pump is connected to is low, the pump stops.

If you have a suction bell in each wet well connected to a common header then your concern is justified. However, if you have flooded suction the pressure will always be above atmospheric. (If not you had better look at the pump's NPSHR (Net Positive Suction Head Required) to avoid cavitation! Since the suction pipe should always be at a pressure greater than atmospheric because of the submergence a simple pressure switch will work. If the wet well is dry and you have air in the suction line, or if the pressure drops below the NPSHR value, then stop the pump. You can use an isolation diaphragm or ring between the line and the switch to avoid plugging the sensing port.