1. #1
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    Default Questions about starting draft

    I've been taught that the tank fill/recirculation valve must be closed when starting a draft in order to prevent air from coming in through that line. Does that apply in all cases, or only when the water level in the tank is below where that line enters? If that line is still below the tank water level, I can't see how you could pull air in through it. Granted in most cases if you're needing to draft you're going to be low on water, but I want to understand the logic a little better.

    Related to this, I'm pretty sure our tank fill valve is not shutting all the way/leaking. I'm not sure how severely, but could this potentially prevent us from being able to draft if we pull to much air through the "closed" valve?

    Also, I've been taught to transition from tank water to draft by slowly closing the tank to pump line and gradually getting the air out of the suction line. However, we have an engine with an air-actuated tank to pump valve, which means it's either all the way on or all the way off. Is there a good way to gracefully transition from tank water to drafting with this setup? The master intake valve is electric, but tank to pump is air.

    Thanks in advance,

    Andy

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    Pelican: Historically speaking, older pumps (1950 & older) had the primer line connected to the highest point in the pump casting. The primer pump was connected to the drive line with a shift collar to engage the primer pump. The main pump did not turn when priming. The entire pump needed to be filled before shifting into "PUMP". If you pulled the tank to pump, the water would simply flow down the suction tube (unless there was a foot valve on the suction) and into the drafting source. Also most tank fills were plumbed into the top of the tank and were exposed to air if the tank were down part way. In the few seconds between prime and pump while making the shift, any air leak allowed the water to drain out of the pump making it impossible to hold the prime.
    Today's pumps have an electric primer pump that is tapped into the pump casting near the eye or eyes of the impeller(s). Any water that is drawn into the pump is immediately thrown into the impeller and forced into the discharge manifold. When the manifold is full, it simply backs up into the impeller, but leaves the center of rotation free of water. This allows the primer to continue to draw air out of the impeller eye. If a tank fill were opened after some pressure was being generated by the pump, the water would simply be directed into the tank, effectively preventing air from coming back the tank fill line.
    The tank to pump line is another matter. As soon as the tank to pump line is opened, the water will flow out of the tank and into the suction manifold. With no water being discharged from the pump (all discharges closed) the tank water will flow down the suction tube and into the pond or stream. Any air trapped in the suction tube will remain there unless evacuated by the primer, and the tank water will flow down the bottom side of the suction tube with little or no effect on removing the trapped air. Under a no flow condition there is no advantage to opening the tank to pump when priming.
    If you can build a small amount of pressure and then open a discharge with the tank to pump line open, you can effectively “entrain” the air in the discharge water and reach a successful prime. There is a problem with this technique of dumping the tank while priming. If your attempt is unsuccessful and the tank is now empty, there is a very good chance that the tank to pump valve doesn’t close completely. Just as you have described in your post about “I’m pretty sure our tank to pump valve is not shutting all the way”. Now it will be necessary to find a source of water to put into the tank to cover the valve and exclude the air from leaking through the valve.
    Every time you pull the pressure down in the pump by ˝ the air doubles in volume. This is the reason that it becomes harder and harder to achieve prime at higher lifts. 1 cu ft. of air at atmospheric becomes 2 cubic feet at 15” of vacuum. At 22” the volume has increased to 4 cubic feet. At 26” that 1 cubic foot has expanded to 8 cubic feet. The primer pump operates at a constant rate of volume per revolution, so it takes longer and longer to reach higher lifts. Small air leaks at high vacuums can completely overwhelm the primer pump.
    If you can get a portable pump into a suction inlet and feed water through the engine and out a discharge, you can successfully prime an engine with a failed primer. You should try this and practice it as a “Just-in-case” scenario.
    When switching from tank water to suction, keep the pump RPM constant. Grab the primer and wait for the air operated TTP valve to close. You might be able to make it smoother by having a recirculation line going back to the suction source and flow it pretty hard thus entraining the trapped suction tube air in the discharge.

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    Quote Originally Posted by Pelican631 View Post
    I've been taught that the tank fill/recirculation valve must be closed when starting a draft in order to prevent air from coming in through that line. Does that apply in all cases, or only when the water level in the tank is below where that line enters? If that line is still below the tank water level, I can't see how you could pull air in through it. Granted in most cases if you're needing to draft you're going to be low on water, but I want to understand the logic a little better.
    It generally applies in all cases. Not because it will draw in air, but because it will draw in air or water. If you're trying to pull a prime, then you need to create a vacuum inside so that atmospheric pressure will push water up into your pump. If the tankfill is open, then it will suck in air or water from there first, rather than through your hard suction hose. When pulling a prime, typically every valve should be closed, except for the intake leading to the static source.
    Quote Originally Posted by Pelican631 View Post
    Related to this, I'm pretty sure our tank fill valve is not shutting all the way/leaking. I'm not sure how severely, but could this potentially prevent us from being able to draft if we pull to much air through the "closed" valve?
    Try doing a dry vacuum test. Do it once with the tank full, and again with the tank empty.
    Quote Originally Posted by Pelican631 View Post
    However, we have an engine with an air-actuated tank to pump valve, which means it's either all the way on or all the way off. Is there a good way to gracefully transition from tank water to drafting with this setup?
    Even air operated valves can be throttled (partially opened or closed). But it's a delicate process than takes practice.

    Does the T2P valve control have three positions? Most do. You know open & closed, but the middle position is called "neutral." That's where the control doesn't push or pull on the valve.

    Say you want to close the T2P valve slowly. Move the control into the middle/neutral position. Then, very quickly, move it to the closed position and back. Repeat as necessary. The problem with this type of control is that air expands. So if you fill the actuating cylinder with a little too much air, it may close the valve anyway, even if you've got the control in the neutral position.
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    What these guys are saying is correct.

    By the way, I use to show recruit pump operators old tricks of the trade, but not to mean that this was it correct way to do things.

    I would take a 1970 model pumper with a Hale 1250 GPM 2 stage pump, hook up two hard sleeves and connect them to the right steamer suction and place the opposite end in the drafting pit. I would connect my 2-1/2" hose lines to the discharges I was going flow water from to a deck gun mounted on the ground at the drafting pit. I would open the gate valves to the discharges, and then run the rpm up to 1400 turns and pull the primer, and wham-oh water would be picked up and then flow from the discharges to the deck gun.

    Folks that had a little more time in the box, that was outstanding pump operators , would stand there and shake their heads in disbelief!

    It was because of the pump and the primer system. Most pump makers say that you must have everything closed to get a draft.
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    Quote Originally Posted by KuhShise View Post
    Today's pumps have an electric primer pump that is tapped into the pump casting near the eye or eyes of the impeller(s). Any water that is drawn into the pump is immediately thrown into the impeller and forced into the discharge manifold. When the manifold is full, it simply backs up into the impeller, but leaves the center of rotation free of water. This allows the primer to continue to draw air out of the impeller eye. If a tank fill were opened after some pressure was being generated by the pump, the water would simply be directed into the tank, effectively preventing air from coming back the tank fill line.
    Quote Originally Posted by txgp17
    It generally applies in all cases. Not because it will draw in air, but because it will draw in air or water. If you're trying to pull a prime, then you need to create a vacuum inside so that atmospheric pressure will push water up into your pump. If the tankfill is open, then it will suck in air or water from there first, rather than through your hard suction hose. When pulling a prime, typically every valve should be closed, except for the intake leading to the static source.
    Ok so let me see if I can reconcile these two statements, since at first glance they seem to contradict. If my tank fill valve is open when I start to prime, I could pull water in, as txgp17 says. However, that water should be thrown right back at the discharge manifold and back out the same way it came in, according to KuhShise. Which wins, the primer or the impeller?

    I also did some playing with the air valve, and was able to get it to throttle partway using the method txgp17 suggested. However, it's darn tricky and I was still never able to throttle it with enough control to switch from tank water to draft without using the primer.

    Finally, another question. I know that NFPA says that you should be able to evacuate the air out of the pump and two sections of hard sleeve within 30 seconds. What is a reasonable amount of time for this plus a dry hydrant with about 6-10 feet of 6" PVC above water level? How long should I let the primer motor run before starting to look for leaks?

    Thanks for all of the help guys, this is great information for someone new at all of this.

    Andy

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    Andy:
    When you are making the switch from tank water to hard suction in a static source, either the trapped air in the suction sleeve must be evacuated by the primer or pass through the pump and be sent out a discharge along with the flow of water. If the flow rate out the discharge is not large enough, the air will migrate to the discharge manifold. It will eventually fill the discharge manifold and the pump volute with the air from the suction sleeve, starving the impeller for water. The discharge pressure will drop (no water in the impeller) and allow the air to be drawn back through the tank fill and break the vacuum that you worked so hard to build. A 12” diameter impeller with a 4” thickness will have about the same amount of air that is contained in 1 1/2 feet of 6” hard sleeve. With two joints of suction, you may need to move 10 volumes of air before you finally get a hard prime. It is certainly easier to get the air out with a primer than trying to beat it into the water and then discharge it out a nozzle. So, Who wins depends upon pump design, single vs 2 stage, Size of discharge, pump wear ring clearance, packing tightness, discharge pressure, pump to tank size and length, nozzle size, pump RPM, suction gaskets seal, etc., etc.
    Some 1500 gpm and all larger pumps require 2 suction sleeves to reach capacity. With a 2 sleeve set-up the primer is required to reach prime in 45 seconds or less. The length of dry hydrant pipe you describe should certainly be evacuated in less than 45 seconds. If you are having a leak issue, shut down the motor and operate the electric primer. Without the noise of the engine it should be possible to hear a substantial air leak in the pump, valves or suction sleeves. Primer pump motors are really starter motors and as such are designed for intermittent use. If you don’t get a prime in under a minute, I’d start looking for leaks. Some of the more modern primer pumps use a solenoid to open the valve between the primer pump and the main pump. Expect to have problems with this arrangement, either from not opening when the primer button is pushed, or to have the valve stick in the open position causing a loss of prime after you start flowing.
    Kuh

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    Never mind, Kuh posted what I was going to say.......... almost.

    FM1
    Last edited by FIREMECH1; 08-04-2009 at 03:01 AM.
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    Our Rosenbauer NH30 two stage pump can have two lengths of hard suction attached but the hose is transparent so we can see the water moving up the hard sleeve whilst the pump is priming.

    I'm not sure about our tanker but i think it has a transparent suction as well. The NH30 has a "Rosenbauer double piston priming pump, automatic operation", thats from our manual.

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    Our low level strainers have 1 1/2" jet assist (jet siphon) on them. When making the connections to draft we connect a piece of hose from a discharge to the low level strainer and charge it. In our experience it pretty much eliminates the need to use the primer because the water is being brought up to the valve by venturi pressure.

    Of course this only works if you have water in the on board tank but it's something to play with. Just another tool in the toolbox.
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