Understanding jet pump failure mode

[Chucky_ott]

I'm trying to understand the failure mode so that I can prevent it again.

The picture shows the discharge port of my jet pump. In the picture, there is a nylon tee and nipple but when it failed, it was a PVC tee and nipple.

For some reason, the entire tee assembly popped out of the pump, spraying whatever water was left in the pressure tank inside the pump room. The pressure switch did not have a low pressure shut-off so the pump kept on working. Don't know for how long but it was still running when I noticed the problem, maybe an hour or so. Maybe more.

On closer inspection of the PVC nipple, it was obvious that it had deformed under heat. I need to understand how.

I'm thinking there are only two ways for enough heat to have been generated to melt the PVC:
1. pump has dead headed because of failed pressure switch
2. pump ran dry because of loss of prime and then dead-headed because cut-off pressure could not be reached

I'm fairly certain the pump did not dead head because of the pressure switch. The switch and all the contacts looked fine so I'm sure the pump shut off at the 60psi cut-off. And when i replaced the tee, it was working properly.

I'm 99% certain that water supply is not an issue. The pump is connected to a 10ft deep sand point close to a lake. Water table is about 1ft deep, pump is 3ft off the ground, so total lift of 4ft. It also has been supplying enough water after I made the necessary repairs.

So the only other possibility that I can think of would be a suction leak. When the pump was off (and the suction pipe was under pressure), there was a slight water leak at the 1-1/4" intake port. I'm assuming that a slight water leak when under positive pressure (pump off) would be enough to admit air when under negative pressure (pump on) and cause the pump to run dry (even if it had been working properly in the hours before). Are my assumptions correct ?

I have since replaced the pressure switch with one with a low-pressure shut-off and replaced the PVC tee with nylon. Luckily, the pump is still working and I have water again. But I doubt that the low pressure shut-off switch will do anything in the event of a suction leak IF NO WATER IS BEING USED. In this case, the system pressure could stay between the low-pressure shut-off and the cut-in of the pressure switch, effectively dead-heading the pump.

Am I correct, and if so, is there a sensor that can detect run-dry conditions for a 120 V jet pump?

 

[Valveman]

Can't see the pump model number? But if the pump, considering the depth of water, only has a max pressure of like 62 PSI, then the pump will get hot trying to get to 60. The pump needs to have a max pressure of at least 10 PSI more than the pressure switch shut off point.

Air will leak from connections that water cannot leak out of. It is nearly always a suction leak. When the pump gets hot, it makes the leak even worse. Don't know of anything 115V to protect that pump. But jet pumps are dual voltage and can easily be set to run on 240V. On 240V you can use a Cycle Sensor to protect the pump from running dry as it uses amps not pressure to make than decision. Because you are correct that a low pressure kill switch will not help unless you are using water at the time of the melt down, which usually is not the case.

 

[Sponsor]

 

[Chucky_ott]

It's a Pompco TJC-50
https://pompco.com/en/jet-pump-pjc.php

The pump doesn't have any problem reaching the 60psi cut-off. It runs at most 30 seconds to fill the 20-gallon tank.

I guess I'm wondering if a minor suction leak would cause a loss of prime during that 30-second run time.

 

[Chucky_ott]

The suction line has 3 unions and 2 barb fittings, and several threaded fittings. That's at least 8 spots for a suction leak. I guess i'll try the shaving cream test to see if there are any leaks. I need to winterize the pipe so I tried to make it convenient. Maybe that was a bad idea.

 

[Chucky_ott]

Would a T&P relief valve on the tee attached to the discharge pump be a good idea? If the temperature rises too much, it might release the pressure enough for the low-pressure cut-off on the pressure switch to activate.

 

[Valveman]

It's a Pompco TJC-50
https://pompco.com/en/jet-pump-pjc.php

The pump doesn't have any problem reaching the 60psi cut-off. It runs at most 30 seconds to fill the 20-gallon tank.

I guess I'm wondering if a minor suction leak would cause a loss of prime during that 30-second run time.

The specs you show for that pump say it can only build 62 PSI max. If it is more than 4.6' deep to water, that pump cannot dependably get to 60 PSI. Even if/when it does, at 60 PSI a 62 PSI max pump is only producing about 2/10's of a GPM. Suction leaks are more pronounced at low flow. At high flow the little bit if air coming in cannot overtake the large amount of water, and you don't lose prime. But at low flow suction leaks come to see you quickly. Most likely the pump can build up to 60 some times, but after using a certain quantity of water the water level in the well drops a little and the pump can no longer reach 60 and shut off. That is when it gets hot. Getting hot causes the pump to be able to build even less pressure, and softens the plastic pipe and makes the leaks even worse.

Shutting that pump off at 50-55 would be much safer. Getting rid of the unions and extra barb fittings on the suction make for fewer possibilities for a failure.

Also a T&P valve will not help. No room to set the pressure adjustment between 60 and max 62 PSI, and by the time the thermal pops at 150F it is already too late.

 

[WorthFlorida]

A T&P for water heaters is 150 PSI or 200 degree water temp. Trying to find an answer on the internet, temps from 175-225F PVC will start to soften.

Commonly done with commercial irrigation pumps in Florida, a pressure relief valve is installed right above the outlet of the pump. These are adjustable but none that I know of works on temperature. I have one of these at the main shut off for my house city water connection. Required now in Florida. https://www.homedepot.com/p/Watts-3...T-Expansion-Relief-Valve-LF530C-3-4/203473942

Where I worked at one time workers for a contractor that was hired to remove trailers needed water. Only a 3hp irrigation pump was nearby. I shut the main valve off and showed them the controller to switch the pump on and off. Well, these clowns left the pump off but didn't open the main valve for irrigation. That night the pump was on (about 6 hours) from the controller for a programmed watering cycle to irrigate about 5 acres. The next day the PVC pipe from the pump to the shutoff valve, about two feet swelled from a 2" pipe to about a 3". However the pump survived.

 

[Chucky_ott]

Shutting that pump off at 50-55 would be much safer. Getting rid of the unions and extra barb fittings on the suction make for fewer possibilities for a failure.

Also a T&P valve will not help. No room to set the pressure adjustment between 60 and max 62 PSI, and by the time the thermal pops at 150F it is already too late.

The pressure switch was actually preset at 30-50 but I had increased it to 40-60 at some point. I'll drop it back down to 30-50. I'll probably also relocate it on the pressure tank tee instead of on the pump where it is now. And of course, I'll address all the fittings.

 

[Chucky_ott]

A T&P for water heaters is 150 PSI or 200 degree water temp.

Something like a temperature management valve used on U/V disinfection units could work. They work at a fairly low temperature (100F) if I remember correctly. I'll have to find the specs. VIQUA 650537, Temperature Management Valve | Trojan Technologies

Or maybe a temperature sensor and a solenoid valve.

The pump does have thermal protection on it though so I assume it would have shut down at some point. I'll try to get some information from the manufacturer.

 

[Chucky_ott]

or a pressure washer thermal release valve or other thermal relief valve. 1/4" MPT would fit nicely on the pump's drain plug

Thermal Relief Valves (ultimatewasher.com)

 

[Valveman]

The built in thermal protection is for the motor, not the pump.

 

[Valveman]

They make a temp sensor for centrifugal pumps called the "pump stop" I think. But everyone I talk to says they have problems because it works at 110 degress and the actual temp of the building gets that hot. Best to catch a dry run on low amps before anything gets hot.

 

[Chucky_ott]

They make a temp sensor for centrifugal pumps called the "pump stop" I think. But everyone I talk to says they have problems because it works at 110 degress and the actual temp of the building gets that hot. Best to catch a dry run on low amps before anything gets hot.

Thanks, I googled them and found a link to the web site (overheatprotection.com) but it looks like the domain name has been parked. Likely out of business.

Your dry-run sensor is the first thing i thought of yesterday but when i looked on your site, i noticed that it was for 220V only. I might rewire eventually but can't do that at this time. Pump is in a pump house and it only has one 2-wire circuit serving several fixtures.

I sent an email to Briggs and Stratton to see at which temperature their TRV trips
Briggs & Stratton 208673GS Valve - Thermal Relief - Briggs & Stratton Online Store (briggsandstratton.com)

 

[Valveman]

All you need is two hot wires for the 240V as well. Just move the common to one side of a double breaker and you will have 240V. Even 115V needs a third wire for a ground.

 

[Chucky_ott]

All you need is two hot wires for the 240V as well. Just move the common to one side of a double breaker and you will have 240V. Even 115V needs a third wire for a ground.

Yup, but the existing circuit serves multiple fixtures. I'd have to run a new underground cable to get to the pump house. It's on the to-do list.

 

[Valveman]

Run 240V out there and have a sub panel to spilt out the 115V as needed.

 

[Chucky_ott]

Run 240V out there and have a sub panel to spilt out the 115V as needed.

Good idea (I think)!

Thanks for the information you provided so far. The pump manufacturer did confirm the 62psi limit for the pump. Taking that into consideration, it's quite possible there was not a suction leak but rather the pump could not reach the cut-off point and just kept working until it melted the tee. It's been working at 40-60 for the past two years but given it was so close to the limit, maybe something changed.

Do the springs on pressure switches change stiffness over time ?

Anyways, I'll drop it back down to 30-50 or 35-55 and give it a go.

 

[Valveman]

Working that close to the pumps deadhead pressure even barometric pressure could cause that problem. Most likely the water level in the well dropped a few feet and the pump was no longer about to build up to 60. Shut off at 50-55 would give you more cushion, and that pump has been hot, it may no longer build 62 PSI.

 

[Chucky_ott]

Ughhh.

Good news and bad news. Bad news first

Bad news:
The pump deadheaded this morning again. Not sure why exactly. The nylon tee held up fine but the PVC adapter connected to it did not. It popped out again.

Good news:
Last week, I had replaced the pressure switch with one that had a low pressure cutoff. So when the PVC adapter popped out, the pump shut down. At least now, with the pressure at zero, I was able to lower the pressure of the pressure tank to 4 psi lower than the cut in pressure of the switch now at 30 psi.

Up next:
Find the suction leak and reduce the amount of joints as much as possible. And maybe install a thermal protection valve.

 

[Reach4]

You could be getting a partially clogged jet. Do you know how to clean that?

Have you used shaving cream on your suction joints?