Problems & Solutions

Got a technical problem which needs resolving?

Published:  01 September, 2006

F&R’s technical consultant in this problem solving feature is Kevin Mellott, a professional with over 32 years of experience in the field of public safety. Prior to founding ERASE Enterprises, Mr. Mellott was an Assistant Chief in the Department of Public Safety for the City of Pittsburgh, Pennsylvania. Mr. Mellott also served as the City Fire Marshal. Internationallyrecognised as an expert in emergency response and disaster operations, he has been involved in special rescue operations since 1978 and has been selected as the lead instructor for numerous special rescue projects for the US military.

A firefighter asks: “We have been attempting to pump our fire engine -  pumper’s - capacity in a relay pumping operation but have not been able to reach full capacity. Do you have any ideas that would help us improve the volume that we can supply?”
Kevin Mellott’s answer:
Without all the details of how you are trying to achieve the pumps capacity it is bit difficult to answer your question regarding your specific situation. However, I will cover the most common causes of an engineer not being able to reach the maximum volume capacity of his apparatus’ pump in a relay type of situation. There are three main points to consider when assessing this situation. The first is the type of pump involved, is it a single stage (one impeller) or a multi stage (two impeller’s or more).  The second concern is the basic inverted ratio of volume to pressure which is present in any pump design.  The third point is the concept of friction loss and its effect on your attempts to move volume through a fixed size hose line. Regarding the type of pump involved, I am only going to address centrifugal types of pumps as the other varieties (piston, rotary, turbine) are not typically used on mobile fire apparatus for volume purposes.  The centrifugal pump is designed around the concept of an impeller spinning on a shaft. The water enters the eye of the impeller and then is thrown via centrifugal force out from the eye, along the vanes of the impeller and then out through a volute which directs the total output of the impeller to the pump casing discharges. If the pump design is based on one impeller, also known as a single stage pump, the control of volume versus pressure is pretty simple and is controlled by throttle activity.  If the pump design is based on more than one impeller (stage) then there is a transfer valve which allows the pump to operate in either a volume or pressure mode. 
“Typically, in today’s If the water flows from one impeller to the next then it is fire service, the in ‘series’ or pressure mode. When the water is entering all the impellers at the same time and is discharged to the volute from each impeller separately pump is more the pump is in the ‘parallel’ or volume/capacity mode. commonly seen. The concept is that if you only have one impeller you increase speed of the pump shaft to increase your
However, the point pressure, however you sacrifice volume due to the speed of the impeller moving faster which allows less you must know your water to enter and discharge. In modern fire pumps though, the newer designs and sizes of impellers still allows for a pretty good discharge volume with just one impeller.
Have you ever wanted an oracle to tell you where you are going wrong? In the first of a new series, F&R presents some common questions and their resolutions. All questions may be submitted anonymously via our e-mail: info@hisdorset.com
Achieving greater volume:
The concept behind a two stage pump is that you flip the transfer switch to either pressure or volume position depending on what you want for an output factor.  In the pressure position the water enters impeller number one and is discharged into impeller number two. Since the water is already at a higher pressure as it leaves impeller one, going into impeller two, the second impeller’s energy is compounded and the outbound water going into the volute is at a higher pressure then when it left the first stage of the pump. If you place the two stage pump into the parallel or volume position, each impeller is pumping water directly into the volute and main discharge manifold of the pump and you have a greater volume. Typically, in today’s fire service, the large one stage pump is more commonly seen.  However, the point of this fact is that you must know your pump and use it as it is designed. I have seen many firefighters attempting to get maximum volume out of a two stage pump while the transfer valve is in the pressure position! It just doesn’t work that way and knowledge of your pump design is critical. Speaking of valve and control knowledge, let’s also make sure that our relief valve is not set too low. As you should know fire apparatus pumps have a relief valve that is adjustable to ensure that the maximum pump pressure does not go above a set limit (established each time you are operating the pump by setting the valve). This is a great idea to protect firefighters from over pressurization on hose lines. But, I have also witnessed firefighters attempting to increase pump pressure by throttling up the engine to no avail, since the relief valve was set low and it was relieving the pressure increase through its functioning. The point is to know your equipment! All the controls, the capacity, pump design and operational aspects. One last comment from my perspective as a fire ground command officer. I have always said that the largest line you pick up at the end of the fire had better been the first one you laid down upon arrival and that goes for both attack and supply lines. Lay out the largest supply line(s) you have upon arrival at a working fire, you will be very glad you did if the fire intensifies and if you suppress it quickly, good for you and good to exercise the hose too! If you follow these tips, I think you will find that you will do your best when pumping and relaying water supplies. KEVIN MELLOTT.

  • Operation Florian

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