Why Clapper Valve Is No Problem When Two Engines Are Pumping
What happens to the clapper valves in a Siamese supplied by engines operating at unequal pressures?
This question has been kicked around by firemen since, apparently time immemorial. Some pretty weird and wonderful answers have been developed in the process, many of which miss the mark by a wide margin.
One classic, widely accepted, and totally incorrect answer usually is developed somewhat as follows:
“Ideally, each engine should provide the same pressure at the Siamese. But, sometimes one pump operator makes a mistake and the pressures at the Siamese inlets differ considerably. Assume a required pressure at the Siamese of 100 psi, which Engine 1 is providing after deducting a friction loss of 90 psi from its 190 psi discharge pressure. Engine 2 supplies an identical layout, but, due to an error in computation, is pumping at only 150 psi. Thus, Engine 2 cannot provide the required 100 psi at the Siamese.
“As both engines pump, the higher pressure from Engine 1 closes the clapper against the line from Engine 2. At once, the pressure from Engine 2 rises to its discharge pressure of 150 psi, due to the lack of flow in its line, and again forces the clapper to open. Thereupon, the pressure from Engine 2 drops, and the process repeats so long as the two engines pump at different pressures. The violence of the swing in the clapper will vary with the pressure difference, being reduced to a ‘flutter’ if the pressures are nearly equal.”
Discussion called fallacious
It is my opinion, based upon both hydraulic theory and field experience, that the above discussion is fallacious. Let us take a more detailed look at the facts.
In any study of fluid flow through closed conduits, there are several variables which must be considered. The omission of any can lead to error and misconception.
In a particular situation, however, many of these variables may be reduced to constants when suitable ground rules have been established. For example, in this discussion, the fluid is water at ordinary temperatures. Thus, density, viscosity and compressibility are now constants. The conduit is fire hose, so the friction coefficient is established. If a level stretch is considered, the elevation difference becomes zero. With identical layouts from each engine, the diameter and length are fixed.
Two variables remain
Two very important and critical variables remain: friction, or head loss, and quantity of flow. With everything else established, these are inseparable; each depends upon the other, and one cannot be determined without assigning a value to the other. Neither can be ignored!
Now, note carefully that the classic discussion above, as paraphrased from two popular fire service hydraulics texts, omits any discussion of flow and fails to specify the length or diameter of the respective hose lines. These omissions set the trap! Any discussion of pressure conditions during flow in a conduit, which fails to specify the flow, length, and diameter must be considered suspect!
Any standard method for calculating flows and friction losses can be used in this type study so long as the same method is used throughout. For convenience, I shall use the Insurance Services Office pamphlet, “Fire Department Pumper Tests and Fire Stream Tables.” Also, in the discussion following, I shall use a level layout of 300 feet of 2½ -inch hose from each pumper to the Siamese.
Pressure at Siamese changes
One other point: A truly complete analysis should look beyond the Siamese because when the second engine starts pumping, the total system flow will change, thus changing the pressure at the Siamese. But let’s ignore that refinement for now, as we are concerned primarily with the results of both engines working at once.
We now have sufficient data and information to start our analysis. Let us again assume Engine 1 to be pumping at 190 psi, overcoming a friction loss of 90 psi, and providing a residual of 100 psi at the Siamese. Let us also take time to establish the flow being delivered. The given friction loss of 90 psi in 300 feet of 2½ -inch hose equals 30 psi per 100 feet, for which the tables give us a flow of 360 gpm. Engine 1, therefore, delivers 360 gpm to the Siamese.
In the so-called classic discussion, the statement is made that Engine 2, pumping at too low a pressure, cannot provide the required 100 psi at the Siamese. No reason, no proof, of that statement is given. Perhaps we are expected to assume that Engine 2 is supposed to deliver the same flow as Engine 1, and therefore should encounter the same friction loss. Thus, 150 psi discharge pressure less 90 psi friction loss leaves a residual of 60 psi at the Siamese, well below the required 100 psi.
If we make that assumption, the aforementioned trap has snapped shut and we are caught! It is wrong! There is absolutely no reason or basis whatever for making it!
Basic hydraulics law
Let’s take time out to review a very fundamental law of hydraulics. Flow between two points in a hydraulic system depends upon a pressure differential between those two points. The magnitude of that flow also depends upon all of the other variables discussed earlier, but remember that all but flow and pressure have been made constants here.
What pressure differential is available to cause flow in the line from Engine 2? If we use the figures from the first discussion, we find that Engine 2 is discharging at 150 psi, the Siamese residual is 100 psi, and the difference is 50 psi. ‘There is no other value available to us to use as friction loss from Engine 2 to the Siamese!
‘This loss in 300 feet of 2 ½ -inch hose, or 16.66 psi per 100 feet, will move about 260 gpm. Therefore, under the conditions specified, Engine 2 will deliver 260 gpm to the Siamese. Added to the 360 gpm from Engine 1, this yields a total system flow of 620 gpm. Flow and pressure will very quickly stabilize at this point.
True, at the instant engine 2 begins to pump, there will be a momentary surge at the Siamese. ‘True, the effective areas on opposite sides of the clapper differ, so a slightly higher initial pressure is required to open it. But since the areas by no means differ by a factor of 2:3, the full 150 psi from Engine 2 will never reach the siamese. Flow will commence at some lower value, and immediately friction losses absorb part of that 150 psi.
True, once the clapper opens, the pressure from Engine 2 will fall. But it will only fall to 100 psi. It cannot fall below that point unless the flow from Engine 2 exceeds the 260 gpm previously determined. But, the flow cannot exceed that value unless a sufficient pressure differential exists to permit, or cause, such larger flow. No such larger pressure differential exists! So long as the two pump pressures remain as stated, we are locked in to this answer!
Flows differ-not pressures
Basic conclusion: The pressures at the siamese do not differ; the flows delivered to it do! The laws of hydraulics are not vacated. There can still be one, and only one, pressure at one point at one time. That pressure may vary, but it is still one pressure.
The clapper may swing momentarily until the system stabilizes, but this will be for only a short time, certainly not more than a few seconds.
Let us now look at another point. There are three basic types of siamese connections, one having a single clapper to serve both sides alternately, one having individual clappers for each inlet, and one having no clappers. If the popular theory were correct, in the case of no clapper, would we not have a problem of flow alternately back and forth in the line having low pressure, like alternating current? There is no evidence that this will happen.
Again, if the popular theory were correct, would we not have a pulsating flow beyond the Siamese? This doesn’t happen, either.
Pumping test recalled
This writer participated in June 1966 in a test of a special fire protection system installed to permit pumpers to draft from a stream and discharge through siamese connections into a water main from which other engines could take water at remote hydrants. Two engines were used on the test, one a 750-gpm class A, the other a 500-gpm class B. During the test, the larger unit discharged at 145 psi and the smaller at 130 psi, each through two 50-foot lines of 2¼ -inch hose to the Siameses. Hydrant flow obtained was a steady 1150 gpm with no fluctuations.
Someone is sure to protest that the clapper will flutter, because he has seen it happen. OK, no argument. Sure it will happen, but not for the reason given in what I choose to call the classic discussion! Since this argument is erroneous, there must be some other reason. Let us look for it.
First, recall that we allowed for a short period of time to establish steady flow conditions. In practice, that condition is never quite reached, although we come very close to it. Now, recall why we have needle valves on our pumper gages: to dampen out pressure variations from the pump. No engine-driven pump can provide a completely steady discharge. There is always some fluctuation. Just because we can steady the gage needle, don’t think the variations at the pump have disappeared. It is these variations, which cannot quite be eliminated, which cause whatever flutter there may be in the clapper.
Looking beyond Siamese
Earlier, I mentioned the need to look beyond the siamese for an ultimate analysis. Let us again use the layout given above, and add a nozzle with a 1 ½ -inch tip connected directly to the siamese. Let’s assume a desired nozzle pressure of 80 psi, corresponding to a flow of about 600 gpm. Each engine is to supply half the total flow, or 300 gpm.
At 300 gpm, the loss in 300 feet of 2 ½-inch hose, from the tables, is 63.6 psi, which we will round to 65 psi and thereby provide some allowance for losses in the siamese. Thus the required engine pressure is 65 + 80 = 145 psi.
With Engine 1 pumping alone, let us check the results:
Using the formula:
The nozzle pressure of 35.1 psi corresponds to a discharge of about 395 gpm. At this point, we have about 40 psi at the siamese inlets and about 105 psi in friction loss from Engine 1.
Now Engine 2 commences pumping, but due to an error, only at 120 psi. A trial-and-error method must be used to estimate results:
A.Assume a nozzle pressure of 75 psi, and a total flow of about 577 gpm. Also, assume a loss of 5 psi in the siamese and nozzle, and thus an inlet pressure of 80 psi at the siamese. Then:
This is too low.
B.Try NP = 65, and Q = 538 gpm. Siamese inlet pressure is now estimated to be 70 psi. Then:
This checks with our original assumption and under the conditions stated, we may be certain that Engine 1 is delivering about 310 gpm while Engine 2 is delivering about 250 gpm to the layout.
In conclusion, probably the most important point I wish to make is that it is dangerous to try evaluating any hydraulic system without having, and using, all pertinent data. Don’t make assumptions without checking them out to see if they are valid! □ □