Adding a second bilge pump?

[rusty69]

6 minutes ago, Tony1 said:

 

I think Tony said this was not as good a solution as an expansion vessel in the hot water supply, but at least it would save me having to get a second one....

I'm certainly not going to go against anything Tony has already said. He is a master Jedi. I am a mere padawan.

 

What I would say though is check which pump you have. Some of them advise they don't need an accumulator, so if you are in need o a new pump any time soon, that may be an option.

[Tony1]

2 minutes ago, rusty69 said:

I'm certainly not going to go against anything Tony has already said. He is a master Jedi. I am a mere padawan.

 

What I would say though is check which pump you have. Some of them advise they don't need an accumulator, so if you are in need o a new pump any time soon, that may be an option.

 

Mine is a mid-range pump I think, nothing special. But I suspect whatever the make of pump, it will benefit from not being switched on and off every few seconds because the water flow is too slow. And to be honest, most of the time I use the cold water it will be on a slow flow, to make the water tank last longer.  

 

Tony did say an expansion vessel in the cold water supply will help reduce some of the stress on the calorifier, so you're not contradicting his advice. But I think he did say a dedicated expansion vessel in the hot water supply is the ideal-world solution. 

 

 

[rusty69]

9 minutes ago, Tony1 said:

 

Mine is a mid-range pump I think, nothing special. But I suspect whatever the make of pump, it will benefit from not being switched on and off every few seconds because the water flow is too slow. And to be honest, most of the time I use the cold water it will be on a slow flow, to make the water tank last longer.  

 

Tony did say an expansion vessel in the cold water supply will help reduce some of the stress on the calorifier, so you're not contradicting his advice. But I think he did say a dedicated expansion vessel in the hot water supply is the ideal-world solution. 

 

 

I'm gonna defer to Tony. I may have given you duff info. Checking info on the jabsco site, they say:

 

Quote

The accumulator tank should be fitted in the discharge line from the pump, as close as possible to the pump itself (see diagram). The tank will serve no useful function if fitted in the pump inlet pipework.

 

I'm not sure why this should be the case, but they are the experts

[Ken X]

1 hour ago, rusty69 said:

Jabsco say the accumulator tank should be fitted in the discharge line from the pump, as close as possible to the pump itself (see diagram). The tank will serve no useful function if fitted in the pump inlet pipework.

 

 

I'm not sure why this should be the case, but they are the experts

As I see it, the pump inlet from the tank will always be at atmospheric pressure due to the tank breather so the accumulator will never operate.

 

When in the discharge side of the pump feeding the taps, the closing of the taps will cause a raising of pipe pressure until the pump stops. This charges the accumulator and allows it to maintain the discharge pipe pressure when the tap is opened delaying pump activation.

[rusty69]

1 minute ago, Ken X said:

As I see it, the pump inlet from the tank will always be at atmospheric pressure due to the tank breather so the accumulator will never operate.

 

When in the discharge side of the pump feeding the taps, the closing of the taps will cause a raising of pipe pressure until the pump stops. This charges the accumulator and allows it to maintain the discharge pipe pressure when the tap is opened delaying pump activation.

I really meant the " close as possible to the pump " bit. summit to do with pascals law.

[Ken X]

Aah, yep, that makes sense, will have to refer to my old text books.  Bit beyond my pay grade that one😄.

[Tony Brooks]

2 hours ago, rusty69 said:

I'm gonna defer to Tony. I may have given you duff info. Checking info on the jabsco site, they say:

 

 

I'm not sure why this should be the case, but they are the experts

 

No need, I ran mine with no expansion vessel for 20+ years, but it is vital there is no NRV between expansion vessel and calorifier body. It is just what is best for all circumstances.

 

When I phoned Jabsco technical, the answer they gave me re their bypass pumps not needing an accumulator was far from convincing physics wise. I think it may be marketing to save the plastic production line boys a few quid.

 

 

[rusty69]

Just now, Ken X said:

Aah, yep, that makes sense, will have to refer to my old text books.  Bit beyond my pay grade that one😄.

Mine too. Hopefully grand master Tony will chip in soon.

[Tony Brooks]

4 minutes ago, rusty69 said:

I really meant the " close as possible to the pump " bit. summit to do with pascals law.

 

Pascal says pressure is equal throughout a closed system, so as the pressures builds up with the tap closed, that pressure is felt thought the system. I don't think it will matter a jot where an accumulator is located.

[rusty69]

3 minutes ago, Tony Brooks said:

 

Pascal says pressure is equal throughout a closed system, so as the pressures builds up with the tap closed, that pressure is felt thought the system. I don't think it will matter a jot where an accumulator is located.

Thats what I thought, so it seemed a bit of an odd statement. I wonder if their reasoning for suggesting the accumulator be close to the pump is for winterisation or ease of pressure setting. I'm not sure what other reason there would be.

Edited February 14, 2023 by rusty69

[Tony Brooks]

As I said, I was singularly unimpressed with Jabsco technical.

[Ken X]

OK, having dug through my hydraulic text books to confirm, the wisdom is that a pump produces "pulsations of energy" in its discharge pipe. These pulsations are best dampened as soon as possible before promulgation through the pipework, hence the location of the accumulator as close to the pump as possible. A bit like water hammer but not as severe. 

My source is big, hydraulic system texts which I used to work on so my feeling is the effect on a low pressure boat water system would be much less severe than the systems I was dealing with.

[Tony Brooks]

21 minutes ago, Ken X said:

OK, having dug through my hydraulic text books to confirm, the wisdom is that a pump produces "pulsations of energy" in its discharge pipe. These pulsations are best dampened as soon as possible before promulgation through the pipework, hence the location of the accumulator as close to the pump as possible. A bit like water hammer but not as severe. 

My source is big, hydraulic system texts which I used to work on so my feeling is the effect on a low pressure boat water system would be much less severe than the systems I was dealing with.

 

Especially as the water pumps normally have at least three or more small pumping chambers, so a minimum of three pulses per rev and each one at low displacement. I know we assume liquids are not compressible but I believe a lot of water has dissolved air in it, so it is slightly compressible so that would help damp the pulsations.

[ditchcrawler]

1 hour ago, Ken X said:

As I see it, the pump inlet from the tank will always be at atmospheric pressure due to the tank breather so the accumulator will never operate.

 

When in the discharge side of the pump feeding the taps, the closing of the taps will cause a raising of pipe pressure until the pump stops. This charges the accumulator and allows it to maintain the discharge pipe pressure when the tap is opened delaying pump activation.

Interestingly in process plants with positive displacement pumps the equivalent but opposite of an accumulator is used on the suction line of the pump right adjacent to the pump to smooth out the liquid flow in the suction lines.

Edited February 14, 2023 by ditchcrawler
Picture added

[Ken X]

I am sure you are right Tony. Some of our radial piston pumps were getting on for three feet in diameter with pipework up to several inches in size so we were shifting a lot of oil and generating many tons of load at the rams. A boat system is never going to induce big pulses of pressure so whilst correct positioning of the accumulator may be theoretically desirable, in practice I suspect locating anywhere will work equally well.

Edited February 14, 2023 by Ken X
Didn't quote.

[ditchcrawler]

5 minutes ago, Ken X said:

I am sure you are right Tony. Some of our radial piston pumps were getting on for three feet in diameter with pipework up to several inches in size so we were shifting a lot of oil and generating many tons of load at the rams. A boat system is never going to induce big pulses of pressure so whilst correct positioning of the accumulator may be theoretically desirable, in practice I suspect locating anywhere will work equally well.

Mine is some way away from the pump and when I fitted an external pressure switch I had to fit a second small one near the pump and switch to stop the switch bouncing open and closed.

[Ken X]

3 minutes ago, ditchcrawler said:

Mine is some way away from the pump and when I fitted an external pressure switch I had to fit a second small one near the pump and switch to stop the switch bouncing open and closed.

Interesting, so there can be measurable pressure waves present in a system. Wonder how much is due to the plastic pipes and seals expanding minutely under pressure and how much is pump generated.

[ditchcrawler]

28 minutes ago, Ken X said:

Interesting, so there can be measurable pressure waves present in a system. Wonder how much is due to the plastic pipes and seals expanding minutely under pressure and how much is pump generated.

Water pumps are positive displacement, every revelation of the electric motor sends 4 or 5 pulses of water down the pipes depending on the number of chambers, However the accumulators most important job is to allow a storage of water between the cut out and cut in pressure of the pump switch.