Using an LDH Manifold

Fireground hose evolutions can take many forms, from the very basic large diameter hose (LDH) supply line with 1¾” attack lines to the extremely complex multi-apparatus combination master stream and handline operation. No matter what type of evolution is going to be in operation, the goal should be to provide the required flow as safely and efficiently as possible.

Various types of appliances and adapters are used to put together hose evolutions. One appliance in particular that is very useful is the large gated manifold, an appliance that distributes water from one LDH supply line on the inlet side to multiple gated outlets on the discharge side.

Manifold Designs
Let’s talk about the design of the manifold (aka portable hydrant). Most manifolds are equipped with a pressure relief valve (PRV). Its main purpose is to guard against pressure surges in the system. The PRV is connected to the manifold body and expells excess pressure that may be created from a shutdown of a line or overpressurization from the pump. The PRV can also be preset to the potential pressure setting of the discharge lines, which in turn helps determine an accurate pump discharge pressure (PDP). To adjust the PRV, pump the required pressure to the PRV/manifold and turn the adjusting bolt on the PRV either clockwise or counter-clockwise until just a trickle of water is discharging from the PRV port.

There are three basic types of large manifolds:

  1. A manifold that features an LDH inlet and wyes off to multiple 2½” gated discharge ports, usually three ports. This manifold is used at the end of the feeder line and supplies multiple discharge lines.
  2. A manifold that features an LDH inlet but also has an LDH gated outlet, which allows the manifold to be placed inline in an LDH system. This in turn allows water to be distributed through its discharge outlets and then sent down the line to another delivery system, such as another manifold or engine. It can also be used as a wye and dead-ended. The most common manifold of this type has four 2½” gated outlets.
  3. A manifold that features an LDH inlet and two LDH gated discharge ports.

Gated Wye Operation
The key to a sucessful gated wye operation with the manifold is twofold: First, it’s important not to commit more discharge lines to the manifold than the water supply will allow. For the most part, this isn’t an issue if small attack lines are used. However, when you start getting into the higher flows that a 2½” attack line or other high-demanding lines create, you must ensure that the water supply will support it. Because of the multiple outlets on manifolds, there’s a tendancy to overextend the discharge capabilities, but it’s really simple: Water in equals water out.

The second part of a succesful gated wye operation: developing the correct PDP for the discharge lines connected to it. When discharge lines are extended from an engine, it is easy to control the pressure on the lines because you’re using the individual valves and gauges. When these lines are extended hundreds of feet from the engine and supplied from one discharge through one supply line, you can only monitor one gauge. This can present a real problem when multiple lines are placed in service. Because of this issue, dialing in multiple lines is more difficult and you can’t expect to be as accurate as you would be if they were directly connected to an engine. The goal should be to provide as close to the required flow as possible and in a safe manner.

Calculating the PDP
The first step to establishing the PDP: Determine the dimensions of the line that the PDP will be calculated from. This can obviously be different from incident to incident, but it’s best to establish a common line that will be used a high percentage of the time. This will create a starting point for the pump operator to get water flowing. I like to use a 200′ line, simply because in most cases it will be long enough to reach the fire.

The next step: Preset the PRV on the manifold to the desired pressure. Since there is the chance of having 1¾” and 2½” handlines from one manifold, as well as other types of discharge lines, the goal is to come up with one pressure that will safely flow all types of lines. Remember: More than likely the individual lines will not be 100% accurate. We are looking for ballpark flows that will provide a required flow safely.

Example: You might find the following three types of discharge lines on one manifold:

  1. A 150-gpm combination nozzle and 15/16″ smooth-bore tip combined into a break-apart nozzle with a nozzle pressure of 50 psi and a friction loss of 35 psi per 100 feet of 1¾” hose.
  2. A 250-gpm combination nozzle with a nozzle pressure of 50 psi and a 1 1/8″ smooth-bore tip used on 2½” hose with a nozzle pressure of 50 psi and a friction loss of 15 psi per 100 feet of hose.
  3. A Mercury portable master stream flowing 500 gpm using 3″ hose with a 1½” tip at 55 psi nozzle pressure with a friction loss of 20 per 100 feet of 3″ hose.

In this case, the target pressure on the PRV should be set at 120 psi. Why? The 1¾” hose at 200 feet actually calculates out to 120 psi. This is probably the most important line to keep to the flow and pressure requirements. With the 2½” handline calculating out to 80 psi and the Mercury master stream calculating out to 90 psi based on the 120-psi pressure setting, they will be over-pumped, creating a higher flow and nozzle pressure. Flow tests can evaluate whether this is an issue, but in this particular situation I think you’ll find that it will not be a problem. It’s one pressure does all.

Setting the PRV at the required pressure for the handlines indicates to the firefighters from its activation that the correct pressure at the manifold has been obtained. With the PRV preset, the pump operator simply throttles up to the set pressure on the manifold, and through communication with the firefighters at the valve, slowly continues increasing the pressure until the PRV starts to activate.

What if the lines attached to the manifold are going to be longer than 200 feet? A simple friction loss chart can indicate what the new PRV setting should be, allowing the PDP and PRV to be readjusted accordingly.

Inline Operations
When the manifold is used in an inline operation to support discharge lines as well as sending the remaining water on through the hose evolution, the PRV concept of calculating the PDP will not work. The reason: The PDP must be able to provide the required flow for the entire operation, which could include a number of different operations through multiple manifolds and/or engines being used.

There’s only one accurate way to establish the PDP for the discharge lines in an inline operation: Place an inline pressure gauge between the discharge port of the manifold and the discharge line to read the required pressure for the line. This will allow the valve controlling the line to be gated down accordingly. This requires adding additional gauges to the manifold because they’re not provided from the manufacturer. The other option is to gate the line down by the feel of the nozzle reaction on the handline itself. An experienced firefighter should know what a particular line should feel like at the required flow. Although this method is not 100% accurate, it’s still a safe way of operating.

Relay Pump Operations
A common water supply problem happens during large-flow, multi-company operations. Usually, the first few units placed in service grab the closest hydrants for their water supply. There’s a good chance that some, if not all, of these hydrants are on the same main, thus drawing down the available water from each other. Often these units are in place to attack the fire and have the available hose, appliances and personnel to get the job done. The initial attack gets started, but eventually gets stifled due to a dwindling water supply.These units are already flowing water and in most cases only need a little bit more to accomplish their goal. This is where a relay pump operation comes into play.

A traditional relay pump operation only delivers water to one pumper. There’s a good possibility that the receiving pumper will have water left over from the relay, especially if it already has its own supply line, and will not be able to share it. This is when a relay/manifold operation can be put into place to supplement units on scene. The manifold allows for multiple supply lines to be connected, supplementing units with the balance of water that they need.

Note: Don’t be fooled into thinking that the 2½” outlets on the manifold will restrict the water-delivery capabilities. Yes, the 2½” is more restrictive than a 5″ outlet, but it is possible to move enough water through the 2½” port to make it work. The key to this operation is to use the same diameter hose to supply the engines. The reason: Water follows the path of least resistance. So if you have an LDH and a small diameter hose, more water is going to move through the larger, making for an unbalanced delivery.

When the pumper supplying the relay makes its hookup at the hydrant, a minimum of two LDH lines should be used to get maximum flow. The pumper should plan on throttling up to the maximum allowed pressure if possible.

After the relay/manifold operation is set up, the PRV can be readjusted to the proper setting, which is indicated when just a trickle of water is coming from the PRV port.

Another Tool in the Box
The large manifold is a tool that you probably won’t use too often. However, if the situation should arise—and it will at some point—the manifold will prove itself as a water distribution appliance worth its weight in gold.