People often ask about two things regarding pilot-operated automatic control valves (ACV) in fire suppression systems. The first question is about calculating friction loss. The second question is about how to set ACVs for redundancy.

When reviewing a minimum friction loss chart to calculate the pressure loss through a pilot-operated ACV, many designers will inevitably have a few questions. The trap is reading the chart for the valve’s friction loss and calling it a day, but the actual number takes an extra step if you want the correct figure.

At first, this may seem confusing, so let’s break it down one step at a time.

First off, what is friction loss? Friction loss is pressure lost while water flows through a pipe or valve due to friction. When you flow large amounts of water, friction will drag down the water flow, but this does not mean it will result in lower outlet pressure values. This means that despite the friction loss, you may not see a lower pressure at your sprinkler heads and hose valves in the fire protection system. 

We know what you’re thinking: Since the friction loss will occur, you would think the outlet pressure from an ACV would be lower than the valve’s pressure setting. ACV pressure setting – friction loss = resulting pressure, right? Wrong. 

To overcome pressure loss from friction, the valve can compensate by opening further to allow more water to flow into the system, allowing the ACV to hold stable outlet pressures despite the pressure loss. The ACV will maintain the set pressure until it is fully open under high flow conditions. So, how do you calculate what the pressure loss will be? 

Friction Loss Calculation Challenges

Let’s look at an example to see where the limit is and how to calculate the impact on your system. To demonstrate, we’ll look at the Body Minimum Friction Loss chart from the Zurn Wilkins ZW209FP specification sheet (Figure 1). For this example, imagine having 175 pounds/square inch (psi) inlet pressure available to a 4-inch pressure-reducing-type ACV expected to flow 1,000 gallons/minute (gpm). The valve in this example is set to 155 psi outlet pressure for the fire protection system. 

The expected friction loss through the 4-inch valve when flowing at 1,000 gpm would be approximately 25 psi. So, the question that often arises is, “Will the 25 psi loss from friction result in the outlet pressure dropping from 155 psi to 130 psi when the system sees that peak flow rate of 1,000 gpm?”

The answer is not always and not in this example.

How can that be? We had the valve set to 155 psi, and then we lost 25 psi. We all remember third-grade math class: 155 – 25 = 130, period. 

The unique aspect about the ACV performance in this scenario is that it can use the excess inlet pressure to overcome the friction loss. Having a 20 psi excess in pressure on the inlet side of the valve (175 – 155 = 20) allows the valve to modulate to let more high-pressure water through to the system to maintain higher outlet pressure so that when friction loss occurs, we still hold stable outlet pressures near 155 psi to make sure the system has enough pressure to effectively fight a fire, and potentially save lives. 

In other words, the excess of 20 psi of inlet pressure can offset 20 psi of friction loss.

Simple, right? So, what happens with the other 5 psi of friction loss? The difference (friction loss – excess inlet pressure above outlet setting) will be the actual loss in outlet pressure when flowing at the peak flow rate. 

In our example, an engineer designing a fire system could expect to maintain an outlet pressure no lower than 150 psi. When friction loss is greater than the excess inlet pressure, we can assume the valve is fully open and use the friction loss graph directly. The inlet pressure, 175 psi, minus the fall-off pressure, 25 psi (at 1,000 gpm), gives the outlet a resulting pressure of 150 psi. The friction loss graph is for fully open valves so that it can be used directly and normally whenever it is assumed that the ACV is fully open.

So, our actual equation looks like this:

Resulting pressure = 

If excess inlet pressure < friction loss pressure, 

then ACV inlet pressure – friction loss

(175 psi – 25 psi = 150 psi, as shown in the  

example)

Else ACV outlet setting

How to Set ACVs for Redundancy

When regulating fire suppression system pressure at a main supply line, often there is a need for redundancy. Two potential system scenarios, in particular, may require redundancy.

The first scenario protects against a valve’s failure to regulate pressure to within a safe range. If there is only one valve, and the valve fails to regulate pressure to within a safe range, there could be an increased risk of failures, leading to potential leaks. A backup ACV is set in series to regulate pressure to keep system pressure within a safe range.

In this scenario, the secondary (redundant) valve is upstream of the primary valve. For this setup to function correctly, the secondary valve’s regulating pressure is set to 20 psi to 25 psi higher than the downstream valve. 

For example, the primary valve is set to 140 psi and the secondary valve is set to 165 psi. In the event the primary or secondary valve fails to operate, the redundancy of having a second valve in series prevents over-pressurization of the system and piping.

How to Set ACVs for Bypass Applications

The second potential pitfall with a single primary ACV application may occur when booster pumps cycle on and off. One solution to this problem is the installation of a parallel low-flow bypass ACV (PRV). The smaller ACV will respond more quickly to low flow demand, preventing the surging of the jockey pump.

To set two valves in parallel with one another the bypass valve is normally set approximately 5-7 psi higher than the high flow primary valve.

Matt Sires is a product manager for Zurn Elkay and has been in the industry for 13 years. Feel free to reach out to Matt or another person on the Zurn Wilkins team if you have questions about automatic control valves.