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control of header flow fanin/fanout

Hello, I'm working with schemes where there are a number of flows into, and out of, a fully liquid filled header (incompressible system). A typical arrangement would be to have, eg. 2 parallel flows in, with control valves maybe level controlling upstream vessels. All the outlet flows have FCs. In order to satisfy the mass balance, there's a pressure controller on the header, that drives the setpoints of some or all of the outlet flow controllers.

Client is considering a situation where FCs are wanted on all flows into and out of the header. Statically, this arrangement will still work due to the pressure controller. However, a dynamic model with reasonably good models of the hydraulic interactions and control devices indicates that the system is fairly wonky. Disturbances result in persistent, nonlinear oscillations (nonlinear due to the valve characteristics), and adjusting tuning doesn't help a lot.

Does anyone have any practical experience with this situation? Should I believe the model, or is the real world better behaved? TIA

Reply to
bruce varley
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Believe the model. Purely flow control, incompressible systems are horrible to control. Adding a pressure control loop does not help, because a control loop needs some process time constant to work with, and your system has a pressure time constant way shorter than any possible control mechanism will have. The pressure control will always be way behind any developing disturbance, in one direction or the other. I would be looking into ways to add artificial compressibility to the system via flow through bellows or bladder devices.

Reply to
John Popelish

Bruce, A system of the type you describe (incompressible header) with all flows in and out flow-controlled, is completely impractical. The speed of response of pressure and flow control systems will likely be much slower than the speed of wave travel in the system (speed of sound) between control points.

However, it is possible to have a liquid-filled header with all flows in and out controlled, as described, if there is in fact adequate compressibility, so as to provide a capability for temporary accumulation of imbalance between inflow and outflow. This would be the case in, for example, a long pipeline or large vessel where the small compressibility of the fluid and the elasticity of the walls can nevertheless give rise to a quite substantial capability for what in a pipeline is called 'line pack'. In those circumstances the closed pipe/vessel and it's pressure control of accumulation are the equivalent of an open vessel and level control. In control-system design terms the principles of design are, therefore, identical, and one needs to determine the hold-up capacity and hold-up lag of the vessel/pipe. Obviously, in a system with small capacity, one would need to shed the pressure control and achieve mass balance by having at least one wild (uncontrolled) flow.

As for your model: if it's rigourous in its modelling of the hydraulics (and don't foget both the compressibility of the fluid AND the elasticity of the walls), then believe it!

Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web:

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Reply to
Kelvin Hales

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