Date: August 6, 2013
THE PUMP BODY – THE UNSUNG HERO
In the last installment of the “Anatomy of a Fire Pump”, Steve Morelan wrote the article “The Heart of the Fire Pump”. In it Steve referred to the fire pump as the heart of the truck. Without the pump there is no FIRE truck, you have only a way of delivering personnel and equipment to the scene but no power to fight the fire. Steve referred to the impeller and impeller shaft assembly as the heart of the fire pump. Using this analogy, the pump casing would be the “body” in relation to the impeller being the “heart”. Just as the human body contains the heart and supports it by supplying passageways for the heart to do its job, the fire pump “body” contains and supports it’s “heart” – the impeller shaft assembly. Understanding the parts of a centrifugal pump and how the body works in relation to the impeller shaft assembly is a vital part of understanding the pumping process.
Pump Theory 101 – All centrifugal fire pumps have the same basic components and functions
Waterous uses 3D modeling, Flow Dynamic software and extensive testing to design the passageways that will ensure pump efficiency. There are two types of passageways in the pump body. They are intake anddischarge. The intake passages supply water to the impeller, while the discharge passages direct water to the discharge valves and ultimately to the hose lines. The design of these passages is critical to the efficiency of the pump. The smoother the passageways move the water, the more efficient the pump is.
It is the “body” of the pump that allows the fire pump to supply continuous water flow. As mentioned above the body passageways provide an inlet and a discharge. As the water enters the eye of the spinning impeller (see diagram) the vanes of the impeller hurl the water towards the exit way increasing the velocity of the water, speeding it up. The action is similar to the way a discus is thrown in track and field. As the athlete spins and releases the discus, the discus’s speed is increased as it is released. It is at this point that the pump body takes over the pumping process again. Looking at the diagram you’ll notice that the impeller is offset in the pump body. It is closer on one side than it is on the other. The opening in the body around the exit way is called the “volute”. The volute is a pathway for the water to follow to the discharge side of the pump. The impeller hurls the water in the direction of the increasing open area. One of the basic rules of water dynamics is that water seeks the path of least resistance and high pressure seeks low pressure.
The water is also directed towards discharge because of a pressure differential. The pressure at the beginning of the volute, where the impeller is closest to the body has a higher pressure than the remainder of the volute so the pressure moves into the larger opening towards the end of the volute and the “stripping edge”. The moving water is stripped off the impeller and directed to the discharge. The body acts as a semi sealed vessel and the moving water is captured on the discharge side of the pump building pressure. The pressure builds in the pump and in the hand line after the discharge valve is opened. When the firefighter opens the bale on the nozzle, the higher discharge pressure in the pump is opened to atmospheric pressure at the end of the hose line. The pressurized water in the pump seeks (flows to) the low pressure at the end of the hand line. Because the discharge pressure will drop due to the water flowing out of the pump the rpms of the engine need to be increased to maintain the pressure.
Pump Theory 102 – All pumps are not created equal.
The pump body also has the critical job of supporting the impeller shaft assembly and keeping it true inside the body. The clearances that need to be held inside the body are as low as .003” – the thickness of a human hair. Quality castings and state of the art machining centers are required to machine the bores and flanges to meet the tolerances required for the pump to operate efficiently for a long period of time.
Pumps are made from a number of materials depending on their use or application. Pumps where weight is critical, the body is made from an anodized high strength aluminum alloy resulting in a light weight durable pump body resistant to corrosion and wear. Anodizing may be a little more expensive but is worth it when working with softer materials. Pumps where strength is the major consideration use ductile or cast iron. Both will deliver the strength required to pass the maximum 500 psi NFPA Hydrostatic Pressure Test (Waterous tests the pumps to 600 psi). Pumps that will be exposed to corrosive/brackish water or foam use a bronze material. With some pumps, such as the portables, there is no choice of what material is supplied. In the larger pump product lines there are options for picking the pump material. Ensure that the material that is specified is the correct material for your application.
In previous “Anatomy of a Fire Pump” installments the subject of two stage pumps was briefly discussed. The two stage pump offers a broader operating range for optimal efficiency when operating in the parallel mode for high volume or the series mode for lower flows at or higher pressures. In some applications, using a large capacity single stage pump to operate one or two hand lines is like using a sledge hammer to drive a tack. In the late 1970’s Waterous developed a patented bronze ball-type transfer valve which is used in the two stage CM series pumps. The ball style valve allows for easier transfer between stages. The ball incorporates a spring loaded seal assembly. Only the front edge of the seal touches the bronze housing while transferring. When debris attempts to keep the ball from turning, the seal compresses the spring. It passes over the debris allowing the valve to transfer with less interference than the drum and canister style used in other two stage pumps.
Waterous CM/CS series pumps have a discharge forward design giving the pumps the lowest possible height and a lower center of gravity, leaving more room for hose reels, hose beds and other equipment. Waterous offers a large number of intake and discharge fittings and manifolds to efficiently plumb the most difficult arrangements. The typical discharge valve layout is vertical. The new Waterous “H Manifold” – (H for Horizontal) offers up to 21 discharge outlets on a typical pump assembly utilizing two manifolds. The H Manifold is a low profile, horizontal manifold mounted above the intake fitting.
The function of the pump body during the pumping process is vital. The impeller shaft moves the water and the pump body builds the pressure inside the pump and supplies the passageways required to deliver the water to the hand lines which makes putting the wet stuff on the red stuff possible. The pump body can get the respect it deserves.
Jay Rought, Sales Manager, Central USA and Canada, has been with Waterous for 31 years. Jay spent his first 29 years in the Pump Service Department providing training using our Mechanic’s Demonstration Units (MDU’s) and in-house Mechanic Seminars. Jay has been the Central USA and Canada Sales Manager for the last two years.