Exploring options for process pump automation

Process control applications are progressively relying on digital automation to provide optimized operation, better diagnostics and enhanced data. However, sometimes there are very good reasons to integrate just the right amount of hardwired controls with these digital platforms to provide the best functionality. This can be the case for all types of process pump automation, where a variety of control and protective relays are available to provide complete hardwired automation, or close integration with other digital controls.

Tending to the workhorses

Process pumps are workhorses of many types of processing and manufacturing industries (Figure 1). They come in many physical configurations and capacities, and they may be operated at fixed or variable speeds. Some pumps are specialized for handling product in hazardous petrochemical locations or hygienic life sciences applications. Others are much more of a commodity and simply pump utility or sump water.

It is common to drive process pumps with electrical motors and three-phase power. Here are some typical functional and protective features needed for industrial process pumps:

• Alternating operation
• Seal failure monitoring
• Electrical monitoring
• Intrinsically safe signaling

For smaller and simpler hardwired systems, it is common to use specialized electrical relays to perform some or all of these functions. Even for larger systems or those employing programmable logic controllers (PLCs) or other digital controllers, there are many reasons to incorporate mission-specific relays for these purposes.

Alternating relays

Many processes employ two pumps for redundancy — only one pump needs to run at any given time, and it does not matter which one it is, but it is important that a second pump be available to operate if the first one fails. This is typical functionality for pumping liquids out of tanks or sumps on an intermittent basis. However, it usually is not desirable to only run one pump as needed while keeping the other in reserve.

An alternating relay is a basic device that takes a signal start command from any hardwired or digital logic and alternately starts one load or the other, helping to equalize runtime between the pumps. The relay may also have indicating lights and a switch so users can override operation to only one of the pumps, making it easier for them to service the second pump.

Seal failure relays

Some types of pumps are submersed into a sump instead of being installed on the outside of a vessel. While this is a convenient installation method and can serve to cool the pumping equipment, it also makes the electrical motor susceptible to water intrusion from leaks.

A seal failure relay works with standard resistive shaft seal leak sensors, and it is easily adjustable to match the sensor type. When the monitored sensor detects water, the relay actuates to indicate the fault. Designers can simply trigger an alarm based on this, or they can perform more elaborate operations, such as stopping the faulted pump and starting another.

Phase-monitoring relays

Three-phase electrical motors used for pump service can experience several types of faults. If a single upstream fuse blows, or if one of the motor windings fails, the motor may continue to operate in a single-phase mode, which is not desirable. Other electrical problems include phase reversal, phase unbalance, undervoltage and overvoltage. Each of these is a sign of electrical system issues, which will likely result in undesirable operation, often leading to larger problems.

A basic phase-monitoring relay may be used to detect some of these issues, while more advanced microprocessor-based versions can detect all these issues (Figure 2). As with seal failure relays, it is good practice to incorporate phase-monitoring relays with automation system designs to alarm and usually stop equipment experiencing electrical problems.

Intrinsically safe signaling