Anybody need any Salamander water pumps

[skier-hughes]

Two here, hardly used
http://cgi.ebay.co.uk/Salamander-ESP-120-3-6-bar-pump-/200593541187?pt=UK_Home_Garden_Bathroom_Shower_Units_PP&hash=item2eb44e8843

 

[Penners]

We bought two [pumps] to use for our new shower unit which has 240 jets. One for cold water and one for hot

240 jets??? :shock:

I'm puzzled by the seller's assertion that you need to buy two Salamander pumps - one for hot and one for cold. Such pumps are readily available with twin impellers, serving hot and cold.

 

[skier-hughes]

The seller - me - made some enquiries prior to purchasing them.
If you have a whole house pumped, as we did, if you buy a twin pump, then each time you turn on the cold water both pumps run. The hot water side runs dry, which doesn't do it any good and the pump fails prematurely.
This can happen if you run the hot to wash your hands, or the cold water for dishwashers/washing machines etc.
We have quite a nice shower in our en-suite, makes up for the lack of a bathroom.

 

[Penners]

each time you turn on the cold water both pumps run. The hot water side runs dry

I'm very surprised at this. I'd have thought that the hot water side would simply short-cycle, with the water already in the pump just going round and round. Are the pumps not fed from a positive head of water?

 

[skier-hughes]

I'm no expert, but they told me when the pressure drops on the cold side, when you turn on the tap, both pumps run the same, so hot would run dry. I'd have thought some form of clutch system would be good, but most likely just adds to the price and wouldn't make it economical. The twin pumps are best used for systems where you continually run both at the same time, ie a shower.

 

[Penners]

they told me when the pressure drops on the cold side, when you turn on the tap, both pumps run the same, so hot would run dry.

I understand what you're saying. I'm just puzzled by it. Is the pump below the domestic hot water header tank? If so, then surely its impeller chambers will never be dry, will they?

I suppose the answer to that question depends on whether there is a shut-off valve in both the hot and cold circuits in the pump, and if so whether that valve is before or after the impeller.

H-m-m-m... anybody know?

 

[skier-hughes]

I suppose it would be below the tank, we positioned then on the same level as the tank, but the impeller would be turning and not able to pump water as no hot tap would be on, so would just build up pressure. I have no idea :lol:

 

[Penners]

I shall be interested if anyone has an expert view on this. If the water is just short-cycling round and round in the impeller, I don't think the pressure would keep increasing.

But I suspect we're both playing a guessing game, here!

 

[chuckey]

As most shower mixers are "thermostatic", on switch on the valve shuts of the cold water (body of valve is cold), as hot water flows and the valve heats up it turns on the cold to get to the correct set point temperature. So on this basis every cold water pump would fail!
One problem with these pumps is that the water flow sensors are not sensitive enough, i.e. you have to run 8 L/M before the pump starts, which means you will need 4'+ of head. Once the pump is running there is enough flow because the water is just running into the pump and not trying to flow through the switched off(non-rotating) impeller.
Frank

 

[Onetap]

I shall be interested if anyone has an expert view on this. If the water is just short-cycling round and round in the impeller, I don't think the pressure would keep increasing.!

Pressurisation systems usually have a pressure vessel and a non return valve. The pump runs and compresses the air in the expansion vessel until it is shut off by a pressure switch. Small draw offs use the water stored in the expansion vessel; frequent starts are bad for the motor since there is a large starting current. Eventually the pressure drops to a point where the pump is started by another pressure switch.

A shower pump would have infrequent starts and operating periods of several minutes, so it shouldn't need such complications.

With no flow, the pressure will increase to that shown on the pump curve. The curve shows maximum pressure at 0 flow and maximum flow at 0 pressure loss.

However, with no flow (dead heading) the energy put into the pump shaft is converted into heat. With prolonged no-flow running on a large pump , the water can boil and burst the pump. I don't think it does the impeller any good and should be avoided. I'd consider fitting one of those automatic pressure by-pass valves to allow water to flow from the discharge back into the suction pipes. I'm sure the manufacturer's instructions cover this.

 

[Penners]

Thanks Onetap. That's exactly what I thought...

...ahem.

 

[skier-hughes]

over my head, I'm none the wiser, does it mean either of us was right?

 

[Onetap]

over my head, I'm none the wiser, does it mean either of us was right?

Dunno; It doesn't run dry, there's water in it but the water doesn't move. Dry running would usually wreck the bearings.
It's usually cavitation that wrecks hot water pumps; the hot water boils at pump inlet, due to the low pressure, and the steam bubbles collapse as they pass to the higher pressure at the pump outlet. This rips pieces out of the impeller.

I don't know whether cavitation would happen, given that there's no flow through the pump. You'd have to ask the manufacturers, but running a pump with no flow is usually something to be avoided.

PS I think you would get cavitation, there would be water circulating within the pump scroll and the energy input would cause the water to get hotter. .

 

[skier-hughes]

ok, think I got that

 

[chuckey]

I have had experience with cavitation on industrial pumps (5kW motors). I understood it occurs when the pump is pumping water faster then its inlet flow, so microscopic vacuum "bubbles" form and when they collapse, they do so at extremely high speed, so bits of the water also reach high speeds and when they hit the impeller, they actually cause a little dent in it. The impellers we used had less then .0001" side clearance, which then caused the metal around the dent to rub off, so the pump pressure started to fall. When a faulty pump was removed, the impeller sides looked as though they had been fired at by a shotgun!!
I would be extremely surprised if domestic pumps would suffer from cavitation, not enough initial pressure, also its cocking off the inlet that does it. Cocking off the outlet, just results in a flywheel made of water, though this could cause very high frictional losses and off course any heat generated by the pumps friction is not being conducted away by the flow of water.
Frank

 

[Onetap]

I would be extremely surprised if domestic pumps would suffer from cavitation, not enough initial pressure, also its cocking off the inlet that does it. Cocking off the outlet, just results in a flywheel made of water, though this could cause very high frictional losses and off course any heat generated by the pumps friction is not being conducted away by the flow of water.
Frank

There's an explanation of it on Wikipedia; http://en.wikipedia.org/wiki/Cavitation#Hydrodynamic_cavitation

There are some impressive pictures of cavitation damage on there as well.

There's usually a line on the pump curve showing the net positive suction head (NPSH) required; the NPSH is the minimum head required at the pump inlet for the flow rate, or you will get problems. The NPSH is the static head, minus the fluid velocity head, minus pressure losses due to friction.

Domestic shower pumps are fairly notorious for cavitation because the water is hot and it doesn't take a lot to drop the pump inlet pressure below the boiling point. The instructions usually specify a minimum static head, a maximum water temperature, 22mm minimum inlet connections and full-bore isolating valves, all of which are frequently ignored. Throttling down/cocking off (not heard that term before) the inlet would reduce the flow by adding resistance and would make cavitation more likely. I've also see (well, heard) it on domestic hot water circulating pumps and told the contractors what the problem was; they hadn't a clue what I was on about either.