Time for an old guy to weigh in on the discussion
I have read with interest some of these comments concerning the use of LDH. There are several entries asking for help with practices that enhance the movement of large volumes of water. TRAINING6604 asked about reducing the LDH to 2 1/2 “, and putting the water into the gated intakes (Pony Suction). This is definitely not the way to connect LDH. Generally, 2 ½ “ pipe is designed for flows up to 250 gpm, at this flow an elbow has about 1.5 psi loss but the losses increase by the square of the flow. (Hazen – Williams formula) Putting 500 gpm through the elbow will cause a drop of about 6 psi per elbow and 1000 gpm will cause 24 lbs loss. Combine this with the fact that most pony suctions have at least three (3) – 90 degree bends you will rapidly negate any advantage that you gained by laying 5” in the street. It is best to attach a 6” X Storz directly to the steamer connection. A combination gate and relay relief valve set at 150 to 180 psi will ensure that the attack engine is able to continue to supply attack lines without interruption while switching over from tank water to the LDH supply. You will need to set the operating point of the relief valve above the highest hydrant pressure that you normally encounter in your system. We have some hydrants in our system that approach 200 psi static, but with increasing fire flow, rapidly drop below our SOP set pressure for relief valves at 180 psi.
Repeated radio chatter by pump operators can be reduced to nearly none by the following actions.
1. Initial attack lines on residential fires (2- 1 ¾ + 2 ½) - Connect the relay engine to the hydrant, but do not place the pump in gear. Allow the hydrant to push right through the engine. Hydrants in the range of 50 to 60 psi at 600 gpm will deliver adequate volume to the attack engine at a distance of 700 feet or more. (Fl=K*Q*Q*L) Fl=1/15 * 6 * 6 * 7 or Fl = 16.8 psi Losses in the relay pump would be about 20 psi so at 600 gpm the attack engine would see at least 14 psi on the intake (compound) gauge. Why place the relay engine in the loop? Just in case there is a sudden increase in the required water. Be aware that the pump will be rotating due to the water flow, and it might be necessary to temporarily stop water flow (attack pumper switches to tank supply for 10 seconds or so) while the engineer shifts into pump gear.
CAUTION! If you place the relay engine in gear, you can expect to generate about 60 to 80 psi above incoming hydrant pressure. Thus discharge pressure would be 140 psi and incoming at the attack engine would be 125 psi. Now the attack engine at idle will be developing 200+ psi making it difficult for the attack crews to control nozzles and hose lines and defeating any pressure governor control or relief valves.
2. Multiple apartment or small commercial structure (10,000 sq ft.) calls for about 1000 gpm (3 – 1 ¾ + 2 – 2 ½) or some similar combination . Place the relay engine in gear, but keep the rpm just slightly above idle and not more than 100 psi discharge pressure. As long as the attack engine has an incoming pressure below 80 psi, the relief valve or pressure governor should be able to control pressure fluctuations.
3. In the event that master streams will be supplied (2 – master streams with 1 ¾” tips) 1600 gpm can be delivered by the relay engine at 150 psi and still have about 40 psi incoming at the attack pumper.
4. Maximizing the relay engine at 180 psi should result in the delivery of nearly 2000 gpm through the 700 feet of 5” hose. If you are running the attack engine, you can carefully take the intake pressure right down to zero without risking failure of the relay. Contrary to some thinking, you can ‘t “suck the intake shut” , but might cause cavitation if you turn the rpm up too high. The intake hose will begin to collapse, but the reduced cross section will cause an increase in friction until the internal pressure in the hose exactly matches the air pressure on the outside of the hose. (turbulence losses in the intake) This high velocity and violent shaking can tear the inner liner out of old or poorly bonded hose, so be careful.
5. Summarizing steps 50 psi = Level 1, 100 psi = level 2, 150psi = level 3 & max (180) psi should cover all the needed discharge pressures for the relay engine. Radio chatter = “Engine 12 boost your output to level 2” (100 psi) and you are done on the radio. You don’t need to bump 10 lbs at a time.