Advanced Sequence Valve Training

Sequence valves rely on consistent pressures to switch them at consistent points in the sequence. Pilot and main line pressure can however, be some of the more temperamental signals to control with. The actual pressure in any line will be dependent on the pipework size and length, including number of bends and small adaptors of fittings, fluid temperatures, fluid viscosity (condition), leakage (component wear), flow line pressure spikes (e.g. accelerating flow into the return line requires the mass of fluid to be accelerated, which will generate a higher line pressure) and what other actuators are operating at the same time.

It's always difficult to know what the exact pressure is inside a line, however, it's not unusual for local pilot flows, bypass flows or main system return flows to create significant local pressures or pressure peaks. It is important, when specifying a pressure sequence valve, to work out what the leakage and drain flows will be for each valve in the system. These values can be checked against the pipe sizes to make sure that local flows and pressure drops will not exceed or interact with the switching pressures.

It's not common for companies to publish the graphs for the range of pressures that their valve springs switch from one condition to another. For example, a valve may start to switch at 5 bar and finish switching at 10 bar. One company may quote the catalogue value for this valve as a 5 bar spring and another quote it as 10 bar spring. This probably won't make too much difference unless you have another pressure control valve that switches close to this pressure as well, and may induce and interaction between the two.

Again, catalogue values are quotes at particular operating conditions e.g. flow, temperature, pressure, viscosity etc. Some conversion value will need to be applied to these values if your system is working under different conditions.

Sequence valves the design feature are similar to those of pressure relief valves and have already been covered in the relief valve training section.

The main design features revolve around how many port connections there are and where the pilot are sensed from, either internally or external ports.

The image shows a section through three different sequence valves. Valve A is a direct operated, normally closed valve with the main signal pilot pressure on the end of the spool at 1 and the spring drain reference pressure linked with port 2. Valve B is another direct acting, normally closed valve with internal pilot and external drain. Valve C is a normally closed pilot operated valve with internal pilot and external drain B.

The key to specifying a pressure sequence valve is understanding exactly what flows and pressure you will see in your own circuit. Will the main line flows be consistent and stable enough to use or will you need to include additional pilot and drain lines to provide a suitable signal.

The selection of sequence valve will depend entirely on what logic you are trying to achieve with your circuit.

Pressure vs flow data for sequence valves is not always readily available in manufacturer datasheets. The first graphs show the flow provided for a 2 way valve against the pilot pressure signal it receives. The second curve is the pressure drop across the valve at a particular flow.

This information allows designers predict the flow they will get for a given input signal and the PD losses they will see when the valve is operating. The losses must be less than the supply pressure minus the load pressure.

The second graphs show flow and pressure drops for a 3 way valve, note the curves supply information for the valve when operating in both configurations.

Make sure you have good clearance between the sequence valve set pressure and any relief or reducing valve pressures. Don't forget that when valves are open and relieving, you may have significant pressure drops in pipework and fittings between different valves.