Water Pressure!

[RJD393]

Hi all - I am looking at a possible replacement for my FW pump. Jabsco list 2 different versions of each of there models. one rated at 25psi and the other at 40psi. What is the advantage of a higher psi - it seems more likely to create leaks!!

 

[Sinbad1951]

Two disadvantages of the higher pressure are leaks at joints and the possibility of the water pressure being too high for your pressure relief valve in the calorifier. This will effectively discharge your fresh water supplies into the bilge. If you have a PRV in your system you need to check at what pressure it discharges. Best to have the lower pressure pump. You will find water flow just as effective.

 

[vyv_cox]

I've been struggling to think of a reason why a 40 psi pump might be needed. The only one I can come up with is that it might be needed if there was a fine filter somewhere in the system. My Seagull IV has a fairly low flow rate that might be improved by high pressure. I have never had a pump that did not have a 25 psi pressure switch.

 

[David2452]

When using the small bore tube found aboard small vessels there is an advantage in using a higher pressure pump as it helps with the more important component of flow (not unlike a 24v system being able to deliver the same amps using smaller conductors). Leaks should not be an issue at 45 PSI in a well installed system but the one contra indication to the higher pressure is, as has been mentioned is the PRV, why so many of them are 3 bar is beyond me as most cylinders have a SWP well above that. If you want better flow and better shower pressure then check the SWP of your calorifier and upgrade the PRV if safe to do so, they are cheap as chips. Also worth not restricting yourself to looking at the pump pressure, look at the flow rate too.

 

[RJD393]

Thanks for the replies - the system has a 3bar PRV which should be OK in a 40psi system though a 4bar would be better. There are no unusual filters or small bore pipework so I am pleased no one (so far!) can give me a good reason to use the higher pressure pump.

 

[David2452]

By small bore I meant 15mm as opposed to 22mm or 28mm so I bet you do

 

[theoldsalt]

Thanks for the replies - the system has a 3bar PRV which should be OK in a 40psi system though a 4bar would be better. There are no unusual filters or small bore pipework so I am pleased no one (so far!) can give me a good reason to use the higher pressure pump.

3bar is approx 42psi so if a 40psi pump is installed almost certainly the prv will open each time the pump operates due to pressure pulses in the system each time the pump cuts in and out. Similar to the "bonking" (careful with responses) sound sometimes heard in a home domestic pipe system each time a tap is turned off

 

[nigelmercier]

... one rated at 25psi and the other at 40psi....

I'd like to know where pump manufacturers get their figures. I've got a 3 bar shower pump at home, the shower almost takes your skin off. I've also got a 3 bar pump on the boat (4 bar PRV) and I'm lucky if I can get the shower to spray upwards.

 

[David2452]

I'd like to know where pump manufacturers get their figures. I've got a 3 bar shower pump at home, the shower almost takes your skin off. I've also got a 3 bar pump on the boat (4 bar PRV) and I'm lucky if I can get the shower to spray upwards.

Like I said earlier in the thread it's not just about pressure but flow and pressure combination, in your case both manufacturers are giving valid numbers against a closed head.but have different flow rates.

 

[ghostlymoron]

Bonking in its polite sense is water hammer usually caused by a pressure wave set up when flow is interrupted suddenly. Cross country water distribution mains can generate enormous pressures and are usually controlled by a large 'surge vessel' close to the location where damage can be caused.
Surge vessels come in 2 forms. One has a membrane which separate s the air from the water and prevents absorption of the air. The other has no membrane and has to be recharged occasionally to replace the absorbed air. On a boat you could have either type but it's function is to provide a reservoir of pressure to stop the pump coming on every time you clean your teeth.

3bar is approx 42psi so if a 40psi pump is installed almost certainly the prv will open each time the pump operates due to pressure pulses in the system each time the pump cuts in and out. Similar to the "bonking" (careful with responses) sound sometimes heard in a home domestic pipe system each time a tap is turned off