In its basic form, a flow control valve can be just and orifice.
In the top symbol, we see a flow control valve with a variable area adjuster shown by the cross arrow through it. Note also how the restriction is shown as two curves. In the lower symbol, the restrictor is shown as two sharp corners. Sharpe edges indicate a higher quality flow control valve with a higher degree of viscosity independence e.g. more stable flow with temperature changes.
The middle symbol also shows a pressure compensated valve because of the arrowhead on the line through the valve. The short T-end to the angled line through the orifice indicates a fixed setting rather than an adjustable setting. The third line shows that the excess flow is diverted into a third line.
Hydraulic symbols have been harmonised in the ISO1219 standard but it will take some time for all of the other interpretations to disappear. The symbols that are shown here give an idea of some of the different variations.
The top symbol shows a curved restrictor but the symbol also has a line with a dot that used to indicate temperature compensation.
The bottom symbol has a side arrow which used to indicate a third bypass drain line.
These are earlier versions of that will no longer be included in the ISO standard and should therefore no longer be used.
This symbol shows a common form of flow control valve that meters the flow in only one direction only, the flow will go through the check valve in the return direction. The symbol also shows an adjustable orifice, temperature compensated restrictor and an arrow head to indicate pressure compensation.
This symbol shows how to control the same flow in both directions i.e. with a single flow control restriction. Four check valves are used in a Wheatstone bridge layout to direct the flow through the same valve, no matter which direction it comes from.
This symbol shows a basic priority flow control valve which is designed to always provide flow to the main priority flow path, up to a pre-set limit, then supply the excess flow to the third line. The straight lines either side of the 3 way 2 position valve show that it uses a proportional spool e.g. as the spool moves over the internal orifice opens gradually rather than instantly, as it would in a directional valve. In the outlet of the main supply line, there is an orifice. As the flow increases the pressure drop across this orifice increases and acts to move the spool across against the spring. This movement gradually opens the third line in the spool, which, reduces the amount of flow supplied to the main, right-hand pipe.
The flow divider or flow combiner symbols are one of the easiest to understand. It shows how the flow will be split from one into two lines, in equal quantities by the orifices shown, or combined from two lines into one in equal quantities. However, in reality, flow dividers can be very complex and one of the most troublesome techniques to achieve reliable, repeatable results.
A proportional flow control valve is one that increases its orifice opening area gradually rather than instantly. These proportional valve symbols show a simple two way, two position spool valve with the proportional spool shown by the two additional lines surrounding the valve element. The first symbol is electrically controlled and the second symbol is controlled by a roller that will be in mechanical contact with another object. The third symbol is a electrically controlled variable orifice device.