etermining Engine Pressures Doesn’t Have to Be a Problem
The pump operator encounters a multitude of hose layout problems on the fireground, but despite their almost infinite variety, he has to come up with an answer to only one question: What is the proper engine pressure?
With one formula, a handful of rules and a few friction loss figures in his head, a pump operator can come up with his engine pressure answer.
The only formula worth remembering, as far as a pump operator is concerned, is the one for friction loss in 211-inch hose, commonly written as:
FL = 2Q2 + Q
With this as a starter, a man can go far. What’s the friction loss in 211inch hose with 600 gpm flowing? Q is the number of hundreds of gallons a minute flowing, or gpm -4100. In this example, 600 -4100 = 6. So substituting in the formula:
FL = 2(6)2 + 6
FL = 2 X 36 + 6 = 78 psi
This formula makes memorizing the friction loss table for 211-inch hose unnecessary. A little mental calculation will do the job for any flow.
Now that we have a good thing going, there’s no need to stop with 211inch hose. We have the key to friction losses for 3 and 311-inch hose by remembering a couple of rules:
- To find the friction loss in 3-inch hose, multiply the friction loss for the same gpm in 211-inch hose by 0.4.
- 2. To find the friction loss in 311inch hose, multiply the friction loss for the same gpm in 211-inch hose by 0.17.
Let’s see how these two rules work with our 600-gpm flow, which had a 78 psi friction loss in 211-inch hose. For convenience in doing the mental arithmetic, we round off the 78 to 80. Thus, for 3-inch hose, the friction loss for 600 gpm is 0.4 X 80 = 32 psi. Again, the rounding off process by the pump operator would make this 30 psi. The AIA friction loss table, incidentally, gives 29.9 as the friction loss.
For the 311-inch hose, the pump operator can follow our second rule, so that for 600 gpm flowing, the friction loss is 0.17X80 = 13.6 psi. The AIA table gives 13.4 as the friction loss.
Don’t be dismayed by the need to multiply by 0.17 in your head. With a number ending in 0, such as 80, figure: 8 X 1.7 = 13.6. Note that in dividing 80 by 10, we have to multiply 0.17 by 10 (or move the decimal point one unit to the right to get 1.7).
If we had a friction loss like 36 to multiply by 0.17 while standing alongside a pumper without paper or pencil, we would break the multiplication into two steps. First take 10 percent of 36.0 by moving the decimal point one unit to the left: 3.6. Now multiply: 0.07 X 36 = 2.52. Adding, 3.6 + 2.5 = 6.1 psi. The AIA table gives 6.3 as the friction loss for 400 gpm in 3)2-inch hose. In doing mental calculations, we would round off either figure to 6 psi, so the minor difference would disappear in fireground computations.
Friction losses, of course, are always expressed in pounds per square inch for 100 feet of hose. If the friction loss is 15 psi and there are 850 feet of hose in the line, then the total friction loss is 8.5 X 15 = 127.5 psi.
Calculating back pressure
Friction loss is only one of our problems in determining engine pressure. Whenever a nozzle is higher than the pump, then back pressure exists. This is the pressure exerted by a column of water as a result of gravity. It is independent of the twisting and turning of the hose or diameter of length nf the line.
Back pressure has a constant measurement of 0.434 psi per foot of height. If a nozzle is 100 feet above the pump, then the back pressure the pump has to overcome is 100 X 0.434 = 43.4 psi. For ease in fireground calculations, we can regard back pressure as 0.5 psi per vertical foot or 5 psi per story in buildings. If the nozzle is lower than the pump, then back pressure becomes negative and is subtracted from the engine pressure.
The third ingredient that goes into engine pressure is the desired nozzle pressure. For hand lines with straight tips, 50 psi is a workable standard pressure for both and 232-inch lines. When fog tips are used, 100 psi is desirable for 132-inch lines and 70 psi is recommended for 232-inch hand lines. When master streams are used, the nozzle pressures are often standardized at 80 psi for straight tips and 100 psi for fog tips.
St. Louis Globe-Democrat photo
The three ingredients, friction loss, nozzle pressure and back pressure, that are combined to determine engine pressure can be written in formula:
EP = FL + NP + BP
Remember that back pressure can be negative. If the nozzle is lower than tbe pump, it is then subtracted instead of added. This formula is observed so automatically by experienced pump operators that we don’t look upon it as an equation that must be memorized. As you learn to pump at proper pressures, the use of the equation becomes as natural as your choice of the right verb form in a sentence.
Here’s how engine pressure is calculated. If the friction loss in a hose is 95 psi, the nozzle pressure is 50 psi, and the nozzle is 30 feet above the ground, we determine the engine pressure first by figuring the back pressure.
30 X 0-5 psi = 15 psi
Substituting in the engine pressure equation:
EP = FL + NP + BP
EP = 95 + 50 + 15 = 160 psi
So much for single hose lines. What about parallel lines that are supplying another pumper or are siamesed into master stream appliances?
The computation of friction losses for parallel lines can be quite simple if the pump operator remembers the friction losses for a few key flow rates. If he doesn’t, he can go back to basic FL = 2Q2 -fQWhen the parallel lines are the same length, the length of one line is the length of the layout. If the lines are of different lengths, then the average length is the length of the layout. If the lines are different sizes, then they must be converted to the same size.
In our explanation, we shall assume that the lines are both 2>2-inches and of equal length. We could do the same thing with larger lines, but operators remember mostly friction losses for 232-inch lines and some for 3-inch lines, but not many.
If the flow through two parallel lines totals 600 gpm, then it follows that 300 gpm are flowing through each line. In parallel lines of equal length and diameter, the friction loss in one line is the friction loss figure needed to compute the engine pressure. If the lines were of different lengths, then a friction loss for the average length of the layout would be used.
In this case, the friction loss for 300 gpm in a 2)2-inch line is 21 psi, so 21 psi is the friction loss per 100 feet of parallel 232-inch hose flowing a total of 600 gpm.
If there were three parallel lines in the layout and the total flow was 600 gpm, then each line would carry one third the water, or 200 gpm. The friction loss for 200 gpm in 232-inch hose is 10 psi, and therefore 10 psi is the friction loss in three parallel lines flowing a total of 600 gpm.
If four parallel lines were used, the friction loss per 100 feet for the layout would be the friction loss for one quarter of the total gpm being delivered.
The table illustrates what we have discussed.
Notice how the friction loss figures 10, 15, 21, 28, 36 and 55 appear in both columns. Note also that the gallonage for the two siamesed 21/2-inch lines is twice that of the single 21/2-inch line with the same friction loss.
Chicago Daily News photo. Dace Form II
This characteristic holds true for friction losses for parallel lines of any size.
The friction loss for parallel lines can be determined by dividing the total gpm being delivered by the number of lines. The friction loss for the parallel line layout will be the friction loss for one line flowing its equal share of the total flow.
If the pump operator can remember friction losses for single 2’2-inch lines flowing up to 500 gpm, he can handle any 2M-inch parallel line problem he is apt to encounter. If he doesn’t remember the friction loss for a specific gallonage, he can resort to the friction loss formula: 2Q2 -jQ to figure the friction loss for the single line carrying its equal share of the total layout flow.
If the pump operator is supplying 3-inch lines and can’t remember friction losses for those lines, he can work the problem as though it involved only 2)4-inch lines. Then he multiplies his answer by 0.4 to get the friction loss for 3-inch parallel lines.
Once the friction loss figure for the parallel lines is determined, multiply it by the number of hundreds of feet in the length of the layout (not the total of the individual lines) to compute the total friction loss for the setup. Adding friction loss for the parallel lines, any back pressure, and 20-psi residual pressure for the receiving pumper in a relay (or nozzle pressure and a friction loss in the appliance) totals up to the required engine pressure.
Losses in appliances
Friction losses in appliances vary acording to the flow. A deluge set that has a 3-5 psi friction loss at 400 gpm might show a friction loss of 8-10 psi at 800 gpm. When using ruleof-thumb calculations for engine pressures, a 10-psi friction loss for oldstyle deluge sets should be added to the computations. For newer design deluge sets, 5 psi is enough to allow for friction loss.
For most ladder pipes, 5 psi will compensate for friction loss in the appliance. Siameses and wyes of modem design have insignificant friction losses and they can be disregarded without any noticeable effect on fire streams. With the older Siameses and wyes, it is customary to use 5 psi as a standard friction loss figure.
Let’s consider the problem of the pump operator who has to supply a ladder pipe with a 1 3/4-inch tip 60 feet above the ground. Two 214-inch parallel lines, each 250 feet long, go to a Siamese, and there is 100 feet of 3-inch hose to the ladder pipe.
Using the rule of thumb, here is how the friction losses are totaled:
214-Inch parallel lines,
250 feet, 800 gpm = 21/2 X 35 = 871/2 = 90 psi
100 feet, 800 gpm = 50 psi
Back pressure, 60 feet = 30 psi
Total is the engine -pressure = 170 psi
Note that no friction loss was calculated for the Siamese. The 871/2-psi friction loss in the parallel lines was rounded off to 90 psi. This adequately compensates for friction loss in the Siamese. If the three friction losses computed had all been in multiples of 5 so that none had to be rounded off, the failure to compensate in this way for the friction loss in the Siamese would have no noticeable effect on the ladder pipe stream.
In rule-of-thumb computations for fire streams, all figures are rounded off to the nearest multiple of 5 for ease in adding mentally and for practical setting of pump pressures. Pump gages are not calibrated any finer than 5-psi intervals, and it is impractical to try to read a pump gage any closer than that because of the slight needle flutter, minor fluctuations in engine speed (rpm) and the slight variations in the angles at which the operator reads the gage.
Note: Friction losses for 211 and one 3-inch line, siamesed, are about 1/4 less than the friction loss for two 211-inch lines siamesed.
Working with wyed lines
When an engine is supplying wyed lines, the operator must know the sizes of the tips on the wyed lines, the nozzle pressure desired and the diameters and lengths of the wyed lines and the feeder line.
Let’s see how we determine the engine pressure when 400 feet of 4inch hose is wyed into two 200-foot 211-inch lines with Ill-inch tips.
Two lH-inch tips at 45-psi nozzle pressure will deliver 250 gpm each. The friction loss in 211-inch hose for 250 gpm is 15 psi. With 200 feet in each wyed line, the friction loss in the wyed lines is 30 psi. The total delivery of the two nozzles is 500 gpm, so that is what must move through the 3-inch feeder line. The friction loss in 3-inch hose for 500 gpm is 21, so the friction loss in the supply line is 4 x 21 =84 psi. Adding, we get:
EP = 45 psi NP + 30-psi FL in wyed lines -|84-psi FL in the 3-inch line = 159 psi.
The operator would pump at 160 psi. Pumping is not accurate to the pound for several reasons, so pumping at the nearest 5-psi mark on the gage is satisfactory.
In solving wyed-Iine problems involving unequal features, much time can be saved by the rule-of-thumb method. First determine the total flow of all nozzles. This total is needed to find out the friction loss in the feeder line from the pump to the wye. Multiply the friction loss figure by the number of hundreds of feet in the feeder line to obtain the friction loss total for that line. Then compute the total friction loss and nozzle pressure in each wyed line.
Now the pump operator must exercise his judgment and do one of four things:
- He can average the wyed line friction loss and nozzle pressure totals and add the average obtained to the supply line friction loss to determine the engine pressure. As a result, one line will have too much nozzle pressure and the other will have too litde. In some cases the differences between the wyed lines are not enough to cause any trouble with nozzle pressures after averaging.
- The pump operator can use the friction and nozzle pressure total for the line that requires the most. This will provide good nozzle pressure for the line that entered into the final figuring and too much pressure for the other nozzle.
- 3.The operator can use the friction and nozzle pressure figures for the wyed line demanding the lowest total. This will still give this line a good nozzle pressure, but the other line (or lines) will have insufficient nozzle pressure.
- 4. The good operator will use good judgment. If neither line has any special problems and the difference in the required pressures is not too great, averaging the needs of the wyed lines will be a practical answer in calculating engine pressure. On the other hand, if the line that requires the most pressure is in serious need of the extra water, then the operator might consider the second choice above. But he has to make certain that when he gives the most demanding line everything it needs, the line that needs less pressure is not overwhelmed. Fire fighting conditions will influence the operator in compromising the different needs of the wyed lines. This procedure can be used for any number of wyed lines.
The paradox about relay pumping is that the harder you work to perfect it, the less likely you are to be successful. If the engines are spaced so that each is required to do the same amount of work in terms of engine pressure, that should be regarded as a coincidence and not as a virtue.
The key word in the previous sentence about engine pressure is “required.” It is efficient and practical to have each engine, except the one at the fire, start a relay with the same engine pressure. This eliminates mental calculations by the pump operators, who have only to provide the previously chosen pressure for all relay work in their department or mutual aid system.
The standard engine pressure for relays should be selected in relation to the diameter and amount of hose carried on apparatus and the amount of water the relay is expected to supply in gallons per minute. Most relays are set up with a single line because the amount of hose available is limited and the distance between the fire and the water source is often several thousand feet.
When 211-inch hose is used, the flow is generally in the 200 to 250 gpm range. The gpm can range up to 400 gpm with 3-inch hose, but with 351inch hose, 500 gpm can be supplied at about the same engine pressures and distances between pumpers as 200 gpm with 251-inch hose. Or to look at it the other way, an engine pumping through 351-inch hose can supply a line nearly six times the length of a 251-inch line it can supply with the same gallons per minute.
Los Angeles City F.D. photo
The virtues of hose for relay work multiply as the diameter increases. Four-inch hose is about 11 times as efficient as 251-inch hose in terms of waterway capacities, and 5 and 6-inch hose, about 32 and 80 times as efficient, respectively.
When the size hose and the average amount carried on apparatus for relays is known, then a standard engine pressure can be established for all pumpers in the relay except the one at the fire.
If 251-inch hose is used (no apparatus carrying more than 1,800 feet of it), then a delivery of 200 gpm to the fireground is acceptable. A starting engine pressure of 200 psi can be established as standard operating procedure for all engines in the relay except the one at the fire. The latter will operate at the proper pressure for the lines it supplies as though it were operating from a ‘hydrant. The flow, of course, is limited to 200 gpm, but this can be handled through one or more lines as long as the total demand of the nozzles does not exceed the supply from the relay.
Each pump receiving water in a relay should have an intake (residual) pressure of 10 to 20 psi. If suction relief valves are used, they are set to operate and relieve intake pressures in excess of 10 psi. Without the suction relief valves, which are generally used on larger-diameter, single-jacket hose, a minimum intake, or residual, pressure of 20 psi is desirable to ensure a constant supply of water to each pump.
With this information in mind, consider a relay with engines spaced not more than 1800 feet apart supplying 200 gpm. The friction loss will be about 180 psi with approximately 20 psi left for intake pressure at each pump. If the fireground engine has 1,800 feet of hose out in a single 252inch line with a 1-inch tip, then this engine must pump at 230 psi (180 psi for friction loss in the line and 50 psi for nozzle pressure).
Engine pressures should not exceed the annual hose test pressure of the department as a matter of reliability and safety in the relay.
Other standard engine pressures for relays can be determined in a similar manner for other hose sizes and water supply demands.