Why Does My Pump Throb?

[simon43]

My hotel in Phuket obtains water from a well, and we use a standard pump to fill up a water holding tank at a height of about 12 meters.

The water from that tank is then brought down via pipe to ground level again by gravity, and is passed through a second pump with expansion cylinder, then via a large filter to remove any sand/debris, and finally is routed to the showers in each of the 10 guest rooms.

When there is a demand for water from one of the showers, the pump/expansion cylinder combination switch on automatically, then off again when the demand stops. The well pump is also automatic to refill the header tank.

Now here is the problem.

When one is taking a shower, the water from the pump/expansion tank is continuous all the time, at a gravitational pressure, but the higher pressure that one expects from the pump/expansion tank only occurs every 5 seconds or so, for perhaps a second. But that pump is operating continuously whilst there is the demand from the showers.

My question is - why does the pump/expansion tank combo produce a 'pulsed' water output, with 1 second of high pressure and then 4 seconds of low pressure? Is it some characteristic of pumps that use an expansion tank? Or have I configured this all wrong?

Simon

[WarpSpeed]

Yes as pressure in the (expansion) tank drops the pump turns on to replenish as the static pressure (by gravity) is not enough to trigger the pressure switch or if it were it would run all the time regardless of usage and eventually burn out your pump and melt your plumbing..

I hope that's what you're asking and this is helpful.

Personally though since it's gravity fed I would have had the ground pump located after the filter so that it is drawing in only clean water and causes less wear and tear on your pump and the possibility of getting debris and sand caught in the impeller impairing both pumping volume and reliability..

Edited December 15, 2010 by WarpSpeed

[lannarebirth]

Cavitation perhaps?

http://www.engineersedge.com/pumps/cavitation.htm

[WarpSpeed]

I doubt it, cavitation causes a loud scratchy noise while pump is in operation and low flow not a pulsating flow.. That would be starvation but also not likely on a gravity fed system..

[farang62]

Filter the water before the storage tank, install the pressure pump from the storage tank add a final filter after the pump 10 micron should do.

The pressure flirtations you are referring to are cause by NO AIR in the pressure tank. If you have a pressure pump with the tank on the bottom, turn off the pump (power), close the inlet valve to the pump, open a tap to relieve the pressure. On the bottom of the tank there should be a plug/cap opposite the water outlet pipe. Slowly open the cap, take it off (don't loose the rubber seal ring)let all the water out of the tank. Put the cap back, close the tap you opened to relieve the pressure.Open the supply tap from the storage tank, turn on the pump.The pump should run and turn off automatically. Slowly open the pump outlet.

Remember you can not compress water, so you compress the air in the tank, this air pushes the water our, when the pressure gets low the pump starts the process again, results constant smooth flow of water.

Depending on the usage of water this may have to be done once a week or so. If it happens often the pump is probably to small to provide sufficient water. To prevent this problem it is best to use a separate pressure tank, especially for commercial use.

[simon43]

Hi Farang62, thanks for your reply. FYI, the pump has a separate 60 litre pressure tank, supplying water to 10 guest rooms.

I'll follow your instructions and see if it resolves the problem

Simon

[WarpSpeed]

Filter the water before the storage tank, install the pressure pump from the storage tank add a final filter after the pump 10 micron should do.

The pressure flirtations you are referring to are cause by NO AIR in the pressure tank. If you have a pressure pump with the tank on the bottom, turn off the pump (power), close the inlet valve to the pump, open a tap to relieve the pressure. On the bottom of the tank there should be a plug/cap opposite the water outlet pipe. Slowly open the cap, take it off (don't loose the rubber seal ring)let all the water out of the tank. Put the cap back, close the tap you opened to relieve the pressure.Open the supply tap from the storage tank, turn on the pump.The pump should run and turn off automatically. Slowly open the pump outlet.

Remember you can not compress water, so you compress the air in the tank, this air pushes the water our, when the pressure gets low the pump starts the process again, results constant smooth flow of water.

Depending on the usage of water this may have to be done once a week or so. If it happens often the pump is probably to small to provide sufficient water. To prevent this problem it is best to use a separate pressure tank, especially for commercial use.

So I'm curious? How do you get a smooth flow of water when the pressure has to drop enough to trigger the pressure switch? And then refill while also being depleted again while in use...... There has to be an overlap of pressure drop off and repressure and therefore pumping action (throbbing) and not a constantly smooth flow of water, but more power to ya if you can get that off any pressure system of this sort..

[WarpSpeed]

Filter the water before the storage tank, install the pressure pump from the storage tank add a final filter after the pump 10 micron should do.

The pressure flirtations you are referring to are cause by NO AIR in the pressure tank. If you have a pressure pump with the tank on the bottom, turn off the pump (power), close the inlet valve to the pump, open a tap to relieve the pressure. On the bottom of the tank there should be a plug/cap opposite the water outlet pipe. Slowly open the cap, take it off (don't loose the rubber seal ring)let all the water out of the tank. Put the cap back, close the tap you opened to relieve the pressure.Open the supply tap from the storage tank, turn on the pump.The pump should run and turn off automatically. Slowly open the pump outlet.

Remember you can not compress water, so you compress the air in the tank, this air pushes the water our, when the pressure gets low the pump starts the process again, results constant smooth flow of water.

Depending on the usage of water this may have to be done once a week or so. If it happens often the pump is probably to small to provide sufficient water. To prevent this problem it is best to use a separate pressure tank, especially for commercial use.

So I'm curious? How do you get a smooth flow of water when the pressure has to drop enough to trigger the pressure switch? And then refill while also being depleted again while in use...... There has to be an overlap of pressure drop off and re-pressure and therefore pumping action (throbbing) and not a constantly smooth flow of water, but more power to ya if you can get that off any pressure system of this sort..

[trogers]

So I'm curious? How do you get a smooth flow of water when the pressure has to drop enough to trigger the pressure switch? And then refill while also being depleted again while in use...... There has to be an overlap of pressure drop off and re-pressure and therefore pumping action (throbbing) and not a constantly smooth flow of water, but more power to ya if you can get that off any pressure system of this sort..

This was answered.

If it happens often the pump is probably to(o) small to provide sufficient water. To prevent this problem it is best to use a separate pressure tank' date=' especially for commercial use.

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[Pib]

When one is taking a shower, the water from the pump/expansion tank is continuous all the time, at a gravitational pressure, but the higher pressure that one expects from the pump/expansion tank only occurs every 5 seconds or so, for perhaps a second. But that pump is operating continuously whilst there is the demand from the showers.

Above statement sounds strange, specifically the part about the high pressure occurring every 5 seconds or so (which indicates the pressure pump is kicking for a brief period possibly due to a small water leak somewhere "combined with a water logged tank." Since water/liquids basically can't be compressed, when a small amount of water is used like by a leaking commode bowl valve, leaking faucet/spigot, leaking pipe, etc., the pressure pump would kick on to replace that small amount of water lost from the pipes/expansion tank. And when the water demand significantly increases, like when the shower is in use, a water logged expansion tank would could cause the pump to run continuously providing a constant pressure or the pump running the great majority of the time and only kicking off for a second or so during the shower. Pump design is a player in it's switching cycles.

If the core problem is a water logged expansion tank you'll need to completely drain the expansion tank and allow air back into the tank using the air charging valve on the top of the tank or somehow letting air enter from the water outlet of the tank. Then restart the pump to refill the expansion tank with water, which will also compress the air in the tank creating an air spring on top of the expansion tank water to push water out of the tank when the pump is not running....and with a 60 liter expansion tank with the proper air charge you should get about 15-30 seconds of pump off time before it would need to kick back on assuming only "one" water outlet is going full blast. If you got multiple showers going, commodes being flushed, sinks running, just a bunch of water usage occurring at the same time, then the pump is going to be running a lot even in a properly operating system using a 60 liter expansion tank.

If this expansion tank air recharge temporarily fixes your problem, you now need to find out why it lost its air charge. If it's the bladder-type or diaphragm-type tank the bladder/diaphragm may be ruptured which allows the water and air to mix which will cause the air to be slowly absorbed into the water thereby doing away with your air spring. This process would probably take a couple of weeks after the air recharge although you could probably notice the problem starting to come back the same day you did the air recharge by closely monitoring the length of the pump's on/off cycle. In a expansion tank bladder/diaphragm type design it will hold the air charge for years before needing to add some top-up pressure....but the tank manufacturers usually recommend you check/top- up the pressure once a year....kinda like checking your car tire air pressure occasionally although tires don't hold air as good as bladder/diaphragm tanks. Or if your expansion tank is a plain steel/no bladder/no diaphragm type tank it will have an Air Volume Control (AVC) valve on its side...the valve may not be working properly as the purpose of the AVC valve is to add a very small amount of air to the expansion tank to replenish the small amount of air naturally absorbed into the water....keeps the air volume in the tank at the correct level and prevents the tank from becoming water logged.

A bad AVC on standalone booster type water pumps which have the small non-bladder/diaphragm tank on the bottom of the pump is one of the main causes of such a pump losing its aircharge, and then kicking on and off a lot...or just mostly staying kicked-on with any significant water usage. Usually the rubber diaphragm within the Air Volume Control/Air Charger/Balance Valve (different manufacturers give it different names) develops a small hole (like a hole in an inner tube) which prevents it from working properly/sucking in a small amount of air on each pump cycle to replace the small amount of air naturally absorbed into the water. It normally takes about 2-3 weeks for such a pump/tank design to completely lose its air charge/air spring if the Air Charger valve is defective and then the tank becomes water logged. Turn off the pump, drain the tank using the drain plug at the bottom of the tank to allow air to rush in, turn the pump back on, and you have just air recharged you tank/recreated your air spring....unfortunately you'll only get a few weeks of good operation before the tank becomes water logged again and you'll notice the pump pretty much runs continuously while using any water or it kicks in and kicks off every second or two....you definitely know it ain't working like normal. If the Air Charger valve is defective a new OEM valve costs about 500-800 baht, not including installation labor. They take about 5 minutes to change with no special tools needed...about all you need is a crescent/adjustable wrench.

[Totster]

The pressure flirtations you are referring to are cause by NO AIR in the pressure tank. If you have a pressure pump with the tank on the bottom, turn off the pump (power), close the inlet valve to the pump, open a tap to relieve the pressure. On the bottom of the tank there should be a plug/cap opposite the water outlet pipe. Slowly open the cap, take it off (don't loose the rubber seal ring)let all the water out of the tank. Put the cap back, close the tap you opened to relieve the pressure.Open the supply tap from the storage tank, turn on the pump.The pump should run and turn off automatically. Slowly open the pump outlet.

Great post, this fixed the pulsing water problem I have had for ages. Now the water comes at a constant pressure.

Thanks

Totster

[Pib]

The pressure flirtations you are referring to are cause by NO AIR in the pressure tank. If you have a pressure pump with the tank on the bottom, turn off the pump (power), close the inlet valve to the pump, open a tap to relieve the pressure. On the bottom of the tank there should be a plug/cap opposite the water outlet pipe. Slowly open the cap, take it off (don't loose the rubber seal ring)let all the water out of the tank. Put the cap back, close the tap you opened to relieve the pressure.Open the supply tap from the storage tank, turn on the pump.The pump should run and turn off automatically. Slowly open the pump outlet.

Great post, this fixed the pulsing water problem I have had for ages. Now the water comes at a constant pressure.

Thanks

Totster

Now you should be asking yourself why did the tank lose its air charge? Small water/air leak on top of the tank or manifold assembly? Bad air charger valve?

I had water pulsing problem with my Mitsubishi WP-255Q (pump on top of a small tank design) where I would completely drain the tank, let air back in, turn the pump back on, and I've got constant pressure for about two weeks. After about 3 weeks the very noticeable pulsing water problem was back. Drain the tank again, let air in again, and I'm good to go again for a few more weeks.

I fixed the the problem back in Oct 10 and need to update one of my earlier posts on this subject, but the core problem was a bad air charger valve. On a Mitsubishi WP series type pump it's a black, octagon shaped valve that screws into the top of tank with a small black hose on the valve's other side going to the pump manifold water inlet side. On other manufacturers' pumps which use this tank on the bottom design the valve may be mounted/screwed in the water inlet side of the manifold assembly versus the tank. But regardless, the valve's purpose is exactly the same which is to add/replace a very small amount of air to the tank each time the pump cycles on and off....this valve action replaces the very small amount of air that is naturally absorbed in the water since the tank does not use a bladder/diaphragm to separate the air and water.

When I took the air charger valve apart which consists of nothing more than a plastic housing, rubber diaphragm inside, a screw in check ball assembly, two springs, and another check ball assembly that can't be taken apart, I immediately saw the source of the problem--a small hole in the rubber diaphragm which basically prevented the air charger valve from working and accomplishing its purpose of maintaining the proper air charge in the tank. And since my water pulsating problem seem to reappear faster and faster each time I drained the tank over several months, I expect the hole in the rubber diaphragm started off very, very small allowing the air charger valve to partially work and then the hole got big enough to where the air charger valve was not working at all.

Fortunately, I was able to buy a new air charger valve for 800 Baht at a nearby store that sold Mitsubishi pump repair parts. I put the new valve in and have had constant pressure since Oct. Also from doing a water draw-down tests to see how much water you can slowly get from a faucet before the pump kicks-in to replace the water drawn from the tank, this water draw-down volume has been constant since Oct when doing the test every week or so...this test shows me the problem is "not" slowly coming back because of another problem and the problem was only a bad air charger valve.

If you have a Mitsubishi WP pump and have a pulsating water problem you can usually check for proper air charger valve operation by "not" having to remove/disassemble it. Do this by lightly (very lightly) pressing your finger tip against the valve's air inlet point which is the little nipple screw-on cap on the bottom of the valve. Turn a faucet on full blast to allow the pump to cycle on and off and you should feel a light suction on your finger tip every time the pump kicks on and the suction will go away each time the pump kicks off. That suction which you should feel is that very small amount of air being sucked in to replace the very small amount of tank air that is naturally absorbed into the water. My bad valve had no suction because its internal rubber diaphragm had a hole in it; my new valve had the light suction...it was sucking air in....it was keeping my tank charged with air.