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Flow and pressure

Good pressure may not mean good flow.

Good flow may not mean good pressure.

Consider a dripping mains tap.

If the outlet is blocked with a finger, it will be difficult to hold back the water so the pressure is good.

But then let go and what happens?

It drips again, which put another way means the flow is very poor. The pressure may be high but the flow is properly no more than an eggcup full per minute.

Now consider a small stream.

Let us say that the stream is one metre wide and half a metre deep, and that it is flowing at about half walking pace.

The flow rate is then very high – enough to fill garden water butt in half a second! But what about the pressure? Even at the bottom of the stream it will be only a tiny fraction of that for the dripping tap.

So our two examples demonstrate the extremes.

The dripping tap has good pressure but poor flow.

The stream has wonder full flow but very low pressure.

Static and Dynamic Pressures:

Static pressure is the pressure when no taps are turned on and therefore no flow is occurring.

Dynamic pressure is under flow conditions, when some taps are turned on and the pressure becomes progressively lower as the flow passes further through a system and has to overcome more and more resistance.

Every metre of water pipe adds a “frictional” resistance to flow. Every fitting and every appliance and other item of equipment in the pipework add to that friction and, because it is fluid resistance, faster flow means greater pressure reduction.

With fluid flow, the longer or more complex the system, the greater the amount of energy the fluid has to give up to overcome the friction and this energy loss reflects directly as pressure reduction.

Lower resistance means less dynamic pressure reduction and greater flow.

Greater resistance means less flow.

All fittings and valves offer resistance to flow which increases as the flow rate rises. This means that for every fitting there is a range of flow where pressure losses are acceptable but as flow increases above this, the pressure drop across the fitting rapidly increases with less and less flow rate gain and is no longer acceptable.

It is always recommended that the design flow rate for the valve or fitting be such that it falls in the part of a flow – pressure drop graph which is well before the flow rate line starts to curve away.

A given pressure drop across every fitting or valve results in a fixed flow rate through that valve or fitting. If the flow is increased, then the pressure drop also rises. However this is a process of diminishing returns, whereby flow reaches a point such that each step up in pressure drop produces less and less additional flow, until no mater how much the pressure drop across the valve is increased the flow stays more or less unchanged.

The pressure loss across a whole system for a given flow rate is the sum of the pressure losses of all the pipework and fittings in that system at that flow rate.

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