Pump protection against dead heading 3

[eeprom]

Hello,
I am an EE, and I'm working on a control system to protect centrifugal and vortex pumps from catastrophic failures due to dead heading. I am going to use temperature as a leading indicator of failure, so I intend to install a temperature switch onto the housing of the impeller. From a control standpoint, this is very simple: it is just a thermostat. But given the huge variation in pump sizes and shapes, selecting and mounting a good switch is not so simple.

The best solution I have come up with so far is to use a thermally conductive epoxy to mount to the pump casing. But this would be a bit of a pain to install, and it may not hold up as well as a bolt. Tapping into the casing is out of the question. So I am stuck with the question, "how to mount a switch to the casing of a pump?"

Does anyone know if a switch is already made for this application?
If not, does anyone have any ideas for mounting a switch?

thanks
EE

 

[eeprom]

SNORGY,
Thanks for your suggestions. You bring up a good point of other parts failing prior to casing rupture. The system is clearly not adiabatic.

In my estimation, it is a matter of having a constant source of heat energy to a mass of fluid within a steel casing. The temperature and pressure of that fluid will continue to rise until either the casing is expelling that same amount of heat through convection (steady state), or the pressure exceeds casing. I am making the assumption that the fluid cannot escape around seals.

 

[1gibson]

Well, scratch the differential temperature measurement if there is potential for reverse flow. If heated fluid makes its way back to the suction, differential temperature is obviously no longer an appropriate safeguard.

But on that note, with potential for reverse flow, a centrifugal pump can only maintain the differential pressure of shutoff head. So rather than say a 5 ft section (slug of fluid between pump discharge and valve) heating up and increasing pressure, it will flow back through the pump until suction temp is also elevated, which will decrease your NPSHA, which may lead to cavitation and drop in differential pressure before the pump is able to exceed your pressure rating. Or... Maybe not!

What is the NPSH margin (considering vapor pressure at elevated temp), what are the NPSH characteristics at shutoff, what is the pump Nss, etc. Not really a one size fits all solution, is it?


Unless of course, you monitor differential temperature across the valve! Sufficient distance away that heat conducted through the valve/pipe doesn't allow the readings to converge. I'm running low on what-if's now.

 

[hydtools]

eeprom,
I would not expect the pressure to rise above the impellar capacity to generate pressure. The casing intake is open to suction and some slip or internal recirculation will occur. Pressure above the impellar capacity will force fluid backwards reducing pressure to shutoff pressure, the impellar cannot generate enough pressure to prevent backflow. Temperature will continue to rise. The casing, or other pressure containing parts, will not fail until the temperature reduces the material strength to no longer be able to hold shutoff pressure.
If vapor is formed, the impellar capacity to generate pressure will be reduced. Vapor would push back into the suction line. You may get suction blowback before seeing mechanical failure.
Just thinking out loud.

Ted

 

[PersonalProfile]

May be of interest:

http://www.lawrencepumps.com/Documents/Pump explosions Amplats.pdf

Regards,
Lyle

 

[hydtools]

lylebrown00, interesting document. Serious pump failures.

Their simple stand pipe solution, where applicable, permits pressure relief out the suction side of the pump and prevents pump burst.

Other solutions presented, where applicable, to release pressure and prevent pump burst.

Ted

 

[TenPenny]

I have known of this happening once. The pump was an ANSI pump, something about a 1x2-10, pumping hot water, running at 3600 rpm with a 25 or 30 hp motor, if I recall correctly. A millwright on his rounds noticed the pump was not sounding good, and that it looked discolored - so he went back to his maintenance shop to get his tools. When he came back, the pump casing was gone - the pump had exploded, and the casing had flown across the room.

We all figured that if he had been about 5 minutes sooner, he would have either been killed or seriously injured. It turns out that both the suction and discharge valves were shut, so the pump was generating steam inside the casing, and once the pressure/temperature exceeded the casing limits, it blew.

That's the only time I've ever personally known of such an event, but reading that attachment, it's not as rare as I would have expected.

 

[hydromarine]

sounds like the gaurd fabricators may suddenly become busy after reading these threads

 

[SNORGY]

Well...
Shut down on low or no flow...and it doesn't happen.

 

[eeprom]

One thing to remember is that temperature is a leading indicator of a pressure rise. If you have started to build pressure, you are too close.

 

[hydtools]

eeprom,
is your piping closed on both sides of the pump in your problem?

Ted

 

[SNORGY]

Of course, flow stops first before anything else happens.
Just sayin...

 

[bimr]

In reading the incidents, it appears that many of the pump incidents took place over several hours. There is no explanation as to why no one noticed no flow. One would have thought that the no flow would have shut the plant processing down.

In many industrial settings, such events may have caused other catastrophes to occur prior to the pump failure.

Perhaps the operators need the installation of emergency buttons that must be pushed every minute or two similar to what is used on railroad engines. If the button is not pushed, automatic brakes are engaged and an alarm sounds.

 

[SNORGY]

We might be devoting too much attention to looking for something beyond the obvious here. To me, running a pump at deadhead achieves nothing and only leads to failure. If it was my decision to make, then I would decide that once the flow stops, so does the pump. The rest is just tuning (ramp down, timer, soft start / stop). This has worked 100 % of the time for me to date.

 

[BigInch]

I thought the same thing about shutting the system down from a completely different trip point. It would seem that an alarm clock might be the best device in this case.

What would you be doing, if you knew that you could not fail?