Maintaining Ring Main Pressure

[luke04167]

Currently looking into a new, insulated HW ring main.
For context, this is a minor demand run but of significant length, with the loading being primarily single-user hot hose points with the potential for upto 12-15 active at any one time (say 35cu.m/hr @ 8-10bar).

Obviously there are various options for maintaining sufficient backpressure, and I'm curious as to what the consensus is on what is generally the best solution or what are common mistakes that should be avoided.

For the current system, the site is using a reduced tank return run, coupled with a VSD on the centrifugal pump.
Other sites I have seen combinations of control valves, regulating valves, STD valves, VSDs, pressure relief valves......

 

[1503-44]

As you already know, there are many ways to solve that problem.
The optimum solution depends on the load profile, the demand of water over time.
The highest flow rate,
The average flow rate,
The minimum operating pressure needed at the farthest (hydraulically speaking) user.

First try to closely match a pump curve to your demands. A pump that will supply your demands will produce some maximum pressure at your lowest demand flow and produce a maximum flow at a pressure greater than your minimum required pressure. The key then becomes how much variation you have in those flow rates.
Basically the idea is to supply at the average rate and see if you can select a pump that has a high efficiency at that rate, yet have the capacity to supply at the maximum flow rate while delivering at least the minimum required pressure.
Once you choose a pump, then you can determine if you need a tank to store any flow that the pump alone might not be able to supply during an extended period of high use. If variations in pressure and flow are small, a pressurised "accumulator" tank may be all that's needed..
Pipe alone has, or can be made to have, some amount of storage capacity, so with small variations in flow and large diameter pipe, a tank may not be needed.
Then you see how pressure varies against variations in demand flow.
If the pump produces too much pressure at low flow, than a pressure control valve might be needed. If it produces too much flow at maximum demand, a flow control valve might be what's needed. But a pressure control valve should also ensure that maximum flow can be meet and a flow control valve should ensure that maximum pressure can be meet. If that is not possible, or if there are wide variations with pressure vs flow that make that difficult, but changing the pump's speed will solve, then a variable speed control for the pump might be needed. Variable speed solutions are indicated when flow variations vary in magnitude and time, are wide, there is no well defined average, ie. the demands do not have a normal distribution, and the pressure required at each flow rate varies (at least somewhat) with flow^2.
Then at times, maybe some combination of those options might be best.
The end result should also consider initial, maintenance and running costs.
Once you have identified those potential solutions, then you make your final selection.