What is Time Delay Valve? It’s Types?

In many pneumatic systems, deliberate time delays are required to be introduced between specific operations. These time delays can be achieved using pneumatic time-delay valves. A pneumatic time-delay valve is used to generate or cancel an output signal after a specified interval of time with reference to the application or removal of a pilot signal.

    It has been observed that the combined functions of various control elements can usually produce a new function. A new component can be designed to achieve this new function. The new component can be constructed by the combination of individual elements or manufactured in an integrated configuration to reduce its size and complexity. An example is the pneumatic time-delay valve which is a combination of a pneumatically actuated 3/2-DC valve, an air reservoir, and a throttle-relief valve.

A time delay is needed between the application or release of a signal to the pilot port of the valve and its operation. The time-delay function is obtained by controlling the airflow rate to or from the reservoir by using the throttle valve. Adjustment of the throttle valve permits fine control of time delay between minimum and maximum times. In one type of timer, the 3/2-DC valve can be actuated only after building up sufficient pressure in the air reservoir. In pneumatic time-delay valves, typical time delays in the range 5-30 s are possible. The time delay can be extended with the addition of external reservoirs.

    A time delay in the operation of the 3/2-DC valve can be effected with reference to the application or release of the compressed air as a pilot signal. Accordingly, pneumatic timers can be classified as

1.   On-delay timer

2.   Off-delay timer

   In the on-delay timer, the 3/2-DC valve is actuated after a delay with reference to the application of the pilot signal and is reset immediately after the release of the pilot signal. In the off-delay timer, the 3/2-DC valve is actuated immediately on the application of the pilot signal and is reset only after a delay with reference to the release of the pilot signal.

       Pneumatic timers can also be classified according to the type of pneumatically actuated 3/2-DC valve as:

1.      Time-delay valve, NC-type

2.      Time-delay valve, NO-type

   In the on-delay timer, the 3/2-DC valve is actuated after a delay with reference to the application of the pilot signal and is reset immediately after the release of the pilot signal. In the off-delay timer, the 3/2-DC valve is actuated immediately on the application of the pilot signal and is reset only after a delay with reference to the release of the pilot signal.      

One can think of using timers for the automatic control. However, this option is not generally used. This is because, with the use of timers for getting automatic control, we cannot ensure whether a particular operation in the control system is complete or not. Whereas, with the use of sensors for getting automatic control, we can always ensure that a particular operation is complete before the commencement of the next operation in the control system.

Time-delay valve, NC-type

The constructional views of an on-delay timer (NC-type) in the normal and actuated positions are shown in Figure 1. It can be seen from the figure that the 3/2-DC valve (NC-type) operates in the on-delay mode permanently. But, in some designs, the valve can be operated in the off-delay mode by connecting the check valve in the reverse direction. For this purpose, the ports of the throttle-check valve should be brought out.

Time-delay Valve, NO-type

The function of an on-delay timer (NO-type) is similar to that of an on-delay timer (NC-type) except for the type of 3/2-DC valve used. In the on-delay valve (N)-type), a 3/2-DC (NO-type) valve is used whereas in the on-delay timer (NC-type), a 3/2-DC (NC-type) valve is used. The symbol or an on-delay timer (NO-type) is shown in Figure 2.