June, 2009

When running clothes or dish washing machines, or other water appliances, pipes sometimes make a sound as if being struck by a hammer. This phenomenon, known as "water hammer" is caused by the rapid closing of the appliance's water supply valve. The phenomenon is not limited to washing machines; if water flowing inside of any pipe stops suddenly, its momentum can produce a shock or vibration. At the worst it can cause leakage. If water hammer occurs in plants or water processing facilities, it can adversely affect flow meters, manometers, and other field instruments installed on the pipes. Moreover, it can destroy pumps and other equipment, or lead to leaks and major accidents. In what follows we describe how to employ flow control to prevent water hammer.

It is relatively easy to keep water hammer from occurring in most homes, because many plumbing components are available such as "shock absorbers" and special valves for washing machines. However for industrial plants and processing equipment, water hammer cannot be prevented by attaching simple parts (for various reasons including fluid in the pipes being a substance other than water, or the pipes themselves carrying higher pressures). What is required is a control mechanism so that the amount of flow gradually decreases when the substance stops flowing. Generally, the flow of a substance can be adjusted by controlling the opening and closing of a valve based on controller output from flow meters, transmitters, and other instruments used as sensors. To stop the flow while keeping the valve from shutting off abruptly, a flow controller (that controls the valve to adjust the flow) should be combined with a PLC or other programmable controller (that gradually closes the valve).

As mentioned above, a PLC or programmable controller can be used to control the valve that adjusts the amount of flow so that it gradually closes, but this can be difficult and time consuming to set up. Here we introduce the UT55A and UT52A UTAdvanced digital indicating controllers that can easily carry out this type of control.

Unlike previous controllers, these new products come standard with ladder sequence programs. Now, even applications that traditionally required mini PLC–based timer, relay, and other functions can be handled by a single UTAdvanced (figure 2).

Figure 3 on the right shows the signal that controls the valve. When control starts, the controller's PID function gradually opens the valve, then continually controls it to adjust flow. But when closing the valve, a "gradual down" control sequence is performed so that the control signal does not cut out suddenly.

This type of sequence control requires creating a program called a "ladder program." Based on a sequence diagram (electrical diagram), a ladder program makes it intuitive to picture the relationship between input conditions and output. Ladder programs combine commands with codes for the devices being programmed. Figure 4 below shows an example of a ladder program on the UTAdvanced. You can also refer to the example of this program in figure 5 on the right.

-UTAdvanced UT55A/52A Digital Indicating Controllers