By GERARD J. NAYLIS
Scenario: You have been assigned as one of the pumpers in a relay that is supposed to be pumping 1,500 gallons per minute (gpm). As you set up, you remember the lesson on relay pumping from the pump operator’s class that you took – since you have a 1,500-gpm pump, you know that the design data name plate will give you the correct engine speed in revolutions per minute (rpm) with a pump discharge pressure (PDP) of 150 pounds per square inch (psi) to achieve a flow of 1,500 gpm. When pumping at overload, you are able to increase the PDP to 165 psi. You also recall from the class that the friction loss in five-inch large diameter hose (LDH) is dependent on the volume of water being flowed and the distance of the stretch. A relay flow of 1,500 gpm will create friction loss of approximately 18 psi per 100 feet of hose. When relay pumping, you also remember that you want to deliver the water to the receiving pump with a 20-psi residual pressure to avoid cavitation. This equates to a maximum five-inch LDH hose stretch of 800 feet (eight 100-foot lengths at 18 psi = 144 psi, and a 20-psi residual requires 164-psi PDP).
You open the discharge and begin to flow water. You begin to increase the PDP. For this exercise, you are pumping to a single five-inch LDH. You also remember to watch the intake gauge so that your residual pressure does not drop below 20 psi so that the incoming hose and your pump do not cavitate. You suddenly realize that you are not able to increase the PDP past 120 psi without partially closing the discharge valve; this causes the residual pressure to rise back above 20 psi, but doing this also reduces the flow to well below 1,500 gpm. You also notice that the rpm are approximately 250 to 300 fewer than what the design name plate calls for; obviously, you are not discharging the volume of water that was intended. What is the problem, and how do you correct it?
 |
| (1) The backstep of the pumper that sends water shows a five-inch large-diameter hose (LDH) discharge to which the discharge hose was attached on the officer’s side of the apparatus. (Photos by Scott Roland) |
Answer: The pump is not getting enough water to relay through the pump discharge. If you had the correct volume and if the sending pump was discharging at 165 psi (assuming the length of the stretch is appropriate for the friction loss in the LDH), you should have a residual pressure of 20 psi. So, what could be the cause of the reduced incoming water? First, make sure there are no kinks in the incoming hoseline. A kink in an LDH will significantly reduce the volume of water you are able to flow. Second, check the intake valve to ensure that it is wide open. A partially closed intake valve will prevent the full volume of water from getting into your pump.
Now, let’s assume that there are no kinks and the intake valve is fully open. A third possibility could be that the pressure setting on the intake relief valve (IRV) is set too low; this would cause water to be dumped from the valve before it gets into the pump. However, this is likely not the case in this example because you are flowing water and the residual pressure was well below 100 psi, the lowest setting on the IRV. The problem is still that you are not getting enough water.
 |
| (2) This side view of the LDH discharge shows that the pipe for the LDH discharge is actually a three-inch pipe that would not allow a flow of 1,500 gallons per minute. |
To find the answer, we have to go all the way back to the other pump. I noted that the five-inch hose is connected to an LDH discharge, so you would think that there shouldn’t be a problem. But there is, and you have to find it. Then, you look at the pipe and notice that the five-inch discharge is hooked to a three-inch-diameter pipe – therein lies the problem. From the pump to the discharge orifice, you are trying to move a volume of water a three-inch pipe is not designed or intended to carry. After some questioning, you learn that the sending pumper was retrofitted to carry five-inch LDH and had an LDH discharge connection attached to the rear three-inch discharge. Unfortunately, no one took the time to determine just how much water could come out of this particular discharge.
To overcome this problem, there are two solutions you can employ in the field. First, lay an additional supply hoseline from the sending pumper to the receiving pumper to make up for the difference in volume. A second option would be – if you have a portable manifold with a five-inch discharge – to connect the relay hose to that manifold and feed the manifold with three three-inch hoses or one five-inch hose from the LDH discharge (described above) plus a single three-inch hoseline. Employing one of these two tactics should overcome the problem and provide the operation with the correct volume of water.
A long-term solution would be to upgrade the three-inch pipe to a larger diameter capable of moving the full 1,500 gpm or alternatively arranging for multiple feeds to the LDH discharge. Given the cost factor involved, it may be more cost effective to use a portable manifold for the few times that you may need to conduct a relay pumping operation.
An alternative would be to determine if you could operate with a lower volume of water. By reducing the volume of water you are flowing, you also reduce the friction loss. Just as friction loss quadruples when the flow rate is doubled, the opposite is equally true: When the volume of water is reduced by half, the friction loss is reduced by a factor of four. The friction loss for 1,000 gpm through a five-inch LDH is approximately eight psi. So, if you have only two pumpers, you could move 1,000 gallons approximately 1,800 feet through a single five-inch LDH. This is especially helpful if you have a limited supply of water and have 18 lengths of five-inch LDH. Carefully consider what your flow requirements are, and match them to the available resources. In many cases, the available water supply will dictate the length of your stretch. If you need more water, then a second relay from another source may be your best alternative. Or, you should consider a combination of a relay and a water shuttle, depending on the distance of the next available water source. A shuttle may be necessary until you have sufficient resources on scene to establish a second relay, particularly if the relay will require more than two pumpers.
Also consider the recommended working pressure for LDH. Although the test pressure for LDH is 200 psi for standard hose, most sources recommend that you do not exceed 175 psi at the working pressure for LDH. Therefore, Tables 1 and 2 do not contain pump discharge pressures that exceed the recommended working pressure of LDH.
Knowing your resources and matching them to your needs at the fire scene are important keys to a successful fire attack. Being able to analyze problems during a relay operation and taking corrective action will ensure that you get the water you need when you need it.
GERARD J. NAYLIS is a 43-year fire service veteran working as a firefighter and fire officer in career and volunteer fire departments. He has a bachelor’s degree in fire science from Jersey City State College and a master’s degree in administrative science from Fairleigh Dickinson University. He is a certified fire instructor who has taught throughout the United States, Canada, and the United Kingdom.
Pumping safety
Resource Management for High-Capacity Water Shuttles – Structural Firefighting
HIGH-PRESSURE PUMP OPERATIONS
Fire Engineering Archives
