johnmath said:
A pressure tank system has a cut in pressure and a cut out pressure. The pressure must vary between these two set points for the system to operate or the pump would just run continuously. I would like to be enlightened how it could be otherwise...
You are absolutely correct but your previous post mentioned "I have to say I much prefer my set-up without the pressure tank and which doesn't exhibit continual stopping and starting of the pump and the consequential ramping up and down of water pressure and flow rate."
The two statements are in conflict and this indicates that your friend's system has not been correctly set up.
As per the first quoted text, if the system is set up correctly, the pump will start at the cut in pressure, stop when the cut out pressure is reached and will not restart until the tank reaches the cut in pressure again. The tank also needs to be correctly sized and pressurised for the intended use.
Most pressure switches are set at either 20/40 psi, 30/50 psi or 40/60 psi cut in - cut out. Having higher pressure settings gives less fluctuation...this comes back to the basic fact that you need about 4 times the head to double the flow rate. If someone has a 20/40 psi cut in - cut out pressure setting, they won't have the same pressure consistency that they would have with a 40/60 psi setting.
Taking it further, if they had a variable speed drive and an appropriately sized pump, their pressure tank would most likely be set at about 42-48 psi. This is because if you use a variable speed drive, the tank's pressure must be set to 65% of the maximum system pressure. 1 psi = 6.89 kPa and 1 m head = 9.81 kPa.
A variable speed controller is ideal for household use with a pressure tank because it delivers a soft start and delivers the current use demand flow rate when the cut in pressure is reached.
Most people do not realise that a pressure tank's factory pressure setting can be altered.
You can also have 2 or more pressure tanks in tandem to increase the storage capacity and these tanks can be any size as long as their pressures are the same. The tanks can also be some distance apart, for example, you could have two pressure tanks 30 metres apart to supply the house piping at different points and have the pump's hose/pipe fitted to the common supply pipe at mid point. The tanks can even have a few metres different elevation to each other and not have it matter.
If the tank's pressure falls below the correct pressure, the pump will cycle. Tank's need to be regularly checked to ensure that the pressure setting is correct.
The cut in pressure must be set at 3 psi above the pressure tank's pressure to avoid the tank emptying but a 2 psi recommendation is often given. This means that it is critical that the pressure gauge is accurate, particularly if the person adjusting the pressure decides to sail close to the wind and have less than the recommended 3 psi above the cut in pressure setting.
The tank must be drained before the pressure is adjusted. Many people do not know this, 'adjust' the pressure while the tank has water in it and frequent cycling is the result.
Many people who buy and fit a pressure tank continue to use the standard pressure switch, causing the pump to run on and cycle. The correct cut in - cut out settings must be used.
Frequent cycling is a good way to destroy a pump. A pressure tank system must be correctly set up and doing so will result in energy savings and much less pump wear and tear.
Posted Monday 18 Aug 2014 @ 4:05:03 am from IP
# 
it should be able to reading real power and also the power factor, not just "apparent" power (power uncorrected for power factor).