pressure tank derate or eliminate
pressure tank derate or eliminate
(OP)
An irrigation supply system for a large cemetary has (3) 30hp split case pumps & a 4,000 gallon steel tank (w/AVC system, lead/lag controller, etc.), formerly used to supply & pressurize a sprinkler/drip system at the end of a 1.2 mile pipeline.
A redesign now has a lake at the end of that line, with a new booster pump station drawing from the lake for system pressure; the booster controls open & close a lake fill valve at the end of the original pipeline. We're going to change the supply pumps to (3) 15hp units for better performance at the lower head condition. Meantime the 30's are running with discharges throttled to prevent cavitation, have adjusted the pressure controller so they're either on (when valve is open) or off (when closed), not really cycling like a normal pressure system anymore.
We have no level communication between stations so the plan is to leave the controls this way, question is what kind of tank capacity do we need now? Can we ditch the big tank & AVC, put in a small bladder tank instead? How do we size that?
A redesign now has a lake at the end of that line, with a new booster pump station drawing from the lake for system pressure; the booster controls open & close a lake fill valve at the end of the original pipeline. We're going to change the supply pumps to (3) 15hp units for better performance at the lower head condition. Meantime the 30's are running with discharges throttled to prevent cavitation, have adjusted the pressure controller so they're either on (when valve is open) or off (when closed), not really cycling like a normal pressure system anymore.
We have no level communication between stations so the plan is to leave the controls this way, question is what kind of tank capacity do we need now? Can we ditch the big tank & AVC, put in a small bladder tank instead? How do we size that?





RE: pressure tank derate or eliminate
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: pressure tank derate or eliminate
I was able to measure flows & pressures with the exisiting pumps, selected new pumps based on observed losses plus known current & future system demand (in gallons per day drawn from the lake). One new pump running will do 470gpm @ 89hd-ft(39psi)with the fill valve open, should take care of current demand while running only during off-peak utility rates. As demand grows, depending on power rates, etc., we may run two pumps which will produce a total of 728gpm @ 122hd-ft(53psi). Though unlikely, running all three pumps together would result in 819gpm @140hd-ft(61psi).
In any case, the setpoints will be slightly below the valve-open running condition for ON, slightly above it for OFF (so something like 35psi ON, 42psi OFF in the one pump scenario, no sense wasting energy to build pressure any higher). I'm thinking it's similar to the constant pressure VFD situation, where only a very small bladder tank is needed sort of as a reference . . .
RE: pressure tank derate or eliminate
The way I see the system diagram is,
[supply source] -> [tank] ->
[3 x 15 HP] -> [Pipeline] -> [Lake] ->
[New Booster]-> [irrigation system] -> 470/720/819
Tank_Outflow = pipeline's flowrate (gpm) x pipeline operating time (minutes) = Tank_Outflow (gallons)
Tank_Inflow = supply flowrate gpm x time of availability (minutes) = Tank_Inflow (gallons)
Tank Capacity = Tank_Outflow - Tank_Inflow
You can deduct volumes when pipeline operating time overlaps the supply availability time.
Basically, to size tanks you must integrate supply to that tank with demand from that tank (or a lake). That's just keeping track of the volume in - volume out. Tank size is found from the difference between max and min accumulated volumes.
http://f
You can use the results to size the pipeline assuming various operating scenarios, such as this one which assumes a constant pipeline flowrate, with the pipeline running either on or off. If you have a variable pipeline flowrate, just change the pipeline's flowrate to suit your variable rates.
You can examine operating methods to optimize the configuation. For example, it might be more cost effective to have a constant pipeline flowrate 24 hours a day, which could require smaller and fewer pumps, perhaps a larger tank, but the lesser cost for pumps, plus the ease of operation makes up for the cost of the tank, or other operating cost scenarios of your choosing.
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: pressure tank derate or eliminate
The demand side is the 3 million gallon lake, approximately 1-2 foot fill valve operating differential. The supply is the 3 new pumps, running at constant volume only when the fill valve is open - given the pipeline losses, one only running produces the 470gpm (and supplies enough to keep the lake full at current demand levels running off-peak). So, the new supply pumps will be held off by a timer for rate management, then operated by setpoints on the digital pressure controller. Initially we'll use LEAD only and alternate primary duty.
If the lake level starts to fall behind, we'll decide whether to stay in the single pump mode and disable the time of use controller (for longer run time), or go with 2 pumps running by enabling LAG1 at the same setpoint (which results in the 728gpm flow at the higher head due to higher volume in the same pipeline) and stay on time of use - just a money thing there, more watts per gallon at the higher head but still maybe cost effective given the rate structure . . .
In any case, the big tank is just tee-ed off to the side, water doesn't have to pass through it. I think I understand the tank sizing you describe, but isn't our situation different? We don't have the inflow/outflow - we'd rather be using lake level setpoints instead but no ready link so using the existing pressure controls for convenience/cost saving. I'm thinking the tank needs to supply only enough air cushion to prevent the pressure oscillation that occurs when there's no air available, just not sure what that cushion volume needs to be . . .
RE: pressure tank derate or eliminate
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com