“What a pure blessing it was to have a bath in a tub alone in a room where all you had to do was pump the water, not tote buckets.” — Nancy E. Turner
Somewhere in every process facility, liquids flow through pipes. Sometimes that flow is pulled by gravity. Some facilities push the liquid with compressed gas, despite the hazards. Mostly, however, pumps power liquid flow through pipes.
We have been inventing new ways to pump liquid since the days of Archimedes. Today, the pumps that we encounter most often in process facilities are centrifugal pumps, gear pumps, and air-driven double-diaphragm pumps.
They each have their uses and from a process safety perspective, they each have their hazards.
Centrifugal Pumps
Centrifugal pumps have been around since the end of the Middle Ages. (No, we don’t believe that it was the centrifugal pump that brought about the Renaissance). Their central feature is the encased impeller, consisting of a hub from which vanes radiate. Low pressure liquid flows into the hub and the vanes of the spinning impeller fling the liquid outward. The faster the impeller turns, the higher the discharge pressure is and the greater the flow can be. Usually, an electric motor drives the pump, but steam turbines are sometimes used if the plant has an excess of steam. Even internal combustion engines are used in some cases, especially when electric power is unreliable.
Flow can be controlled by varying the speed of the pump. As the cost of variable frequency drives (VFDs) comes down, this is increasingly common. The most common way to control flow from a centrifugal pump, however, is with a control valve that pinches back flow as required.
The hazards associated with centrifugal pumps are typically those associated with the drivers. Electric motors can be ignition sources and steam turbines can be the source of thermal exposure.
As for the centrifugal pumps themselves, seal leaks around the shaft connecting the driver to the impeller is a hazard when the pumped liquid is hazardous. Two approaches for addressing this hazard are double mechanical seals and “canned” pumps, which eliminate the need for seals altogether.
Then there is the hazard of inadvertently blocking the pump discharge while the pump runs. This results in “deadheading” the pump. The deadhead pressure is the highest pressure that the pump can generate. With single-stage centrifugal pumps, the deadhead pressure is typically not high enough to harm the pump itself or any discharge piping. (That should always be a consideration, nonetheless.) The real harm from deadheading a pump is delayed. Deadheading a pump results in flogging a small volume of liquid with nowhere to go. Seals get damaged and that small volume of liquid heats, sometimes dangerously so.
A common approach to protecting against the hazards of deadheading a centrifugal pump is to provide the pump with an interlock that shuts off the pump if it is not moving any liquid. The interlock can be based on low current or low power (a pump that is not moving any liquid doesn’t draw much power) or on low flow. These interlocks must have a delay; otherwise, the interlock will make it impossible to start the pump.
Gear Pumps
Gear pumps are a type of positive displacement pump. They use two meshed gears. As they turn, the gears pull liquid into the suction side of the pump, filling the cavity in the teeth of the gear. The gears turns outward into a tight casing and when each tooth of the gear gets to other side of the casing, they spit the liquid out to the pump discharge. For a given pump, the flow rate is dependent on how fast the pump is running. The volume that the gears spit out is the same, no matter what the back pressure is, meaning that the discharge pressure can be extremely high. Gear pumps are especially useful when pumping viscous liquids or when the discharge pressure needs to be very high.
While gear pumps have the same hazards associated with their drivers that centrifugal pumps do, the principal hazard associated with gear pumps is their theoretically unlimited discharge pressure. Inadvertently blocking the pump discharge while a gear pump runs immediately results in a pressure spike. In some systems, this pressure spike can exceed what the piping system is capable of withstanding. In fact, this pressure spike can exceed the pressure that the pump casing is capable of withstanding. I once worked at a facility that kept fragments of a gear pump on permanent display to remind us all of this hazard.
Modern gear pumps are equipped with internal pressure relief to protect against the hazard of a blocked in discharge. Unfortunately, there is not typically a means to verify—to proof test—the internal relief, so increasingly, facilities are providing their gear pumps with an external pressure relief, with the discharge from the pressure relief far enough upstream to mitigate the heat buildup that comes with the recirculation.
Air-Driven Double Diaphragm Pumps
The third type of pump that we typically encounter in process facilities is the air-driven double-diaphragm (AODD) pump. Plants often refer to them as Wilden pumps, ARO pumps, or Sandpiper pumps, regardless of who actually manufactured the pump, and despite that Iwaki Air, Yamada, and Graco are also manufacturers of this type of pump.
AODD pumps are another type of positive displacement pump. Compressed gas, usually air, drives a pair of diaphragms. As one diaphragm moves to draw liquid into a chamber, the second diaphragm moves to force liquid out of a second chamber. A pair of ball check valves at the entrance and exit of each chamber assures that liquid flow is only from the pump suction into each chamber, and then from each chamber into the pump discharge.
The ball checks in the pump serve to prevent reverse flow in the line. Because the AODD pump is air-driven, it is easy to use in electrically classified areas because it is not electrical equipment. And because it is driven by compressed air, it is easy to mount it on a cart and use it as a portable pump; providing it with power only requires connecting an air hose.
The primary disadvantages of an AODD pump are that compressed air is an expensive way to power a pump, that the pump does not give the smooth, steady flow of either a centrifugal pump or a gear pump, and that the vigorous pulsations of the pump are both noisy and can damage the piping and equipment to which the pump is attached. Because of this problem with pulsations, many isolate the pump from fixed piping with hoses and install pulsation dampening vessels on the discharge line.
As for a blocked discharge, the AODD pump is safer than either of the other two types. The highest pressure that an AODD pump can generate is the pressure of the compressed gas powering it. When the discharge is blocked, the pump simply stops pumping.
Any Pump Type Can Be Used Safely
No pump type is without hazard, and every pump type has applications for which it is the best choice. That said, AODD pumps generally pose the least concern in terms of process safety, when their use is justified. On the other hand, gear pumps have the greatest potential for harm, when their hazards are not properly mitigated.
As for centrifugal pumps? They have been the workhorses of the process industries for centuries. That is not likely to change, and for good reason.
Any type of pump can be used safely, as long as we understand their hazards and properly mitigate them.
