Code requirements for excess pressure

in domestic water systems

By Ron George, CIPE, CPD
President, Ron George Design & Consulting Services

Why and where do we use pressure reducing valves?

Most model plumbing codes require pressure reducing valves (PRVs) on domestic water systems where the water pressure exceeds 80 pounds per square inch. When higher pressures are exposed to plumbing systems there is the possibility of pipes or china fixtures rupturing and causing injuries to people and water damage to the building. In most cases, high water pressures can waste water, because higher pressures generally force more water out of the faucet or fitting. There are many applications where PRVs may be required to reduce high pressures and to maintain a constant lower pressure for proper operation of a system or piece of equipment.

Examples of applications for pressure reducing valves are:

1. Reducing municipal water pressure from high pressure mains to domestic water systems in areas with pressure in excess of 80 pounds per square inch (psi)
2. Make-up water connections to heating or chilled water piping for systems that operate at a lower pressure
3. On the discharge of fire pumps and domestic water booster pumps to help control system pressures
4. On commercial dishwasher final rinse connections to deliver high temperature hot water at a specific pressure for sanitizing the dishes (usually about 15 psi)
5. On the lower or intermediate floors of high-rise buildings with booster pumps and only one pressure zone (not a good design practice)
6. On water spray mist systems in produce display cases at  grocery stores
7. On any specially designed system or piece of equipment requiring a constant lower water pressure

The model plumbing codes limit the allowable pressure in a plumbing system to 80 psi to prevent damage to plumbing fixtures and equipment from excessive water pressure, which can lead to personal injury, fixture damage, water damage, premature failure of seals, high velocity noise, erosion and water hammer issues.

The 2006 International Plumbing Code (IPC), Chapter 6, "Water Supply and Distribution" states:

604.8 Water-pressure reducing valve or regulator."Where water pressure within a building exceeds 80 psi (552 kPa) static, an approved water-pressure reducing valve conforming to ASSE 1003 with strainer shall be installed to reduce the pressure in the building water distribution piping to 80 psi (552 kPa) static or less.

Exception: Service lines to sill cocks and outside hydrants, and main supply risers where pressure from the mains is reduced to 80 psi (552 kPa) or less at individual fixtures."

604.8.1 Valve design."The pressure-reducing valve shall be designed to remain open to permit uninterrupted water flow in case of valve failure."

604.8.2 Repair and removal."All water-pressure reducing valves, regulators and strainers shall be so constructed and installed as to permit repair or removal of parts without breaking a pipeline or removing the valve and strainer from the pipeline."

The 2006 Uniform Plumbing Code (UPC), Chapter 6, "Water Supply and Distribution" states:

608.2 Excessive Water Pressure. "Where static water pressure in the water supply piping is in excess of eighty (80) pounds per square inch (552 kPa), an approved type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to eighty (80) pounds per square inch (552 kPa) or less. Such Regulator(s) shall control the pressure to all water outlets in the building unless otherwise approved by the authority having jurisdiction. Each such regulator and strainer shall be accessibly located above ground or in a vault equipped with a properly sized and sloped bore-sighted drain to daylight, shall be protected from freezing and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping. All pipe size determinations shall be based on eighty (80) percent of the reduced pressure when using Table 6-5."

608.3"Any water system provided with a check valve, backflow preventer or any other normally closed device that prevents dissipation of building pressure back into the water main shall be provided with an approved, listed and adequately sized expansion tank or other approved device having a similar function to control thermal expansion. Such expansion tank or other approved device shall be installed on the building side of the check valve, backflow preventer or other device and shall be sized and installed in accordance with the manufacturer's recommendations.

"Any water system containing storage water heating equipment shall be provided with an approved, listed adequately sized combination pressure and temperature relief valve, except for listed non-storage instantaneous heaters having an inside diameter of not more than three (3) inches (80mm). Each such approved combination temperature and pressure relief valve shall be installed on the water-heating device in an approved location based on its listing requirements and the manufacturer's instructions. Each such combination temperature and pressure relief valve shall be provided with a drain as required in Section 608.5."

An engineer is often called upon to design a piping system in which high water pressures require the design and selection of a pressure reducing valve. I often see designers and engineers show a symbol on the drawings that resembles a bow tie with a pointed flag to represent a pressure regulating valve. This symbol represents what is commonly referred to as a pressure reducing valve station. The engineer or designer should have the requirements for a pressure reducing valve station shown on a detail on the drawings, and the performance and installation requirements should be spelled out in the plans and/or specifications, along with the acceptable manufacturer's requirements or standards that the valve must meet.

There are basically two types of pressure reducing valves: direct acting and pilot operated. Both types generally have globe or angle style bodies made of brass in smaller sizes and ductile iron in larger sizes.

Direct acting pressure reducing valves typically have a globe style body that acts as a spring loaded check valve. The valve seat connects to a spring in the bonnet portion of the valve. Most of these valves have a diaphragm with a chamber above the diaphram that has a tube or port connected to the outlet of the valve, allowing the outlet pressure to act upon the the diaphragm and the spring. The top of the valve usually has an adjustment bolt or screw under a cap for adjusting the tension on the spring that holds a preset tension on the valve seat.

Direct acting valves sense downstream pressure through the port or tube between the valve outlet and the diaphragm chamber on top of the valve. The pressure on top of the diaphram, coupled with the spring setting, act together to provide a closing force on the valve seat. They generally need about a 10-pound pressure drop on the downstream side to operate; this can cause minor downstream pressure fluctuations. This is the most popular style of pressure reducing valve. They are generally less expensive than pilot operated pressure reducing valves in smaller sizes and will control the downstream pressure within a few pounds of the pressure setting. As a general rule, for every 10- pound increase on the inlet pressure to a direct acting valve there can be a one-pound increase on the outlet pressure. Therefore, these valves may not be the best choice on systems with sensitive equipment requiring relatively constant pressures over a wide range of flows.

The pilot operated pressure reducing valve has the same basic construction as the direct operating valve except that it has a diaphragm chamber with pilot operated controls that act upon the spring mechanism. The pilot operated valve has pressure sensing lines that connect to the discharge and inlet chambers of the valve. As the inlet pressure fluctuates, the diaphragm adjusts the tension on the spring so that the outlet pressure remains relatively constant. The trim on the pilot operated valve has a smaller pressure reducing valve that discharges to the outlet of the main pressure reducing valve. The trim pieces may vary from one manufacturer to another or even from one model to another. This valve relies on the pressure setting of the trim to adjust the pressure on the diaphragm. This causes the spring to be lighter and designed only to hold a minimal pressure against the diaphragm.

If you are planning to install a pilot operated pressure reducing valve in any orientation other than horizontal, contact the manufacturer. They may want to increase the spring tension to hold back the additional weight of the diaphragm if installed in an unusual position. The pilot operated valve has the capability to automatically reduce a fluctuating higher inlet pressure to a steady lower outlet pressure over a range of flows. Consideration should be given to parallel installations or series installations if unusual flow or pressure conditions exist.

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Thermal expansion

Some models of pressure reducing valves have overpressure or expansion relief valves that allow a higher pressure downstream to vent to the upstream side of the valve. Thermal expansion occurs in water heaters as the water is heated after a large draw. In this situation, the pressure gauge on the downstream side of the pressure reducing valve can indicate a higher pressure than the upstream side. Basic models of the pressure reducing valves operate similarly to spring-loaded lift check valves, trapping the downstream pressure. The overpressure relief option allows the higher pressure downstream to vent to the upstream side of the valve. [Note: The pressure will only discharge upstream when the downstream pressure exceeds the upstream pressure. So, if the upstream pressure is 200 psi, the internal relief valve will not discharge back to the upstream side of the valve until the downstream pressure is in excess of 200 psi.

If high pressures downstream can damage a piping system or piece of equipment, a pressure relief valve should be installed downstream of the pressure reducing valve to protect the downstream equipment.]

Model plumbing codes are now requiring thermal expansion tanks or some other means of relieving excess water pressure to eliminate thermal expansion in domestic water systems. The thermal expansion relief on a pressure reducing valve is useless and unnecessary on a water service with a backflow preventer just upstream of the pressure reducing valve. There is no place for the excess pressure to go. Thermal expansion from water heaters became a code issue soon after the requirement for backflow preventers started appearing in the plumbing codes and water utility service.

Domestic water expansion tanks are the most common way of dealing with thermal expansion.
There are several ways to design a pressure reducing valve station. It is usually called a pressure reducing valve station because of the associated isolation valves, pressure gauges, unions and the by-pass piping arrangement. In most cases and in critical areas, it is considered good design to provide a by-pass around the pressure reducing valve station with a globe or throttling type valve for manual operation.

Two-stage or series pressure reduction is used in systems that have extremely high inlet pressures or high pressure differentials.

This type of pressure reduction reduces the velocity of the water across the valve seat, which can reduce the life expectancy of the valve, and it reduces the amount of noise generated by the valve. Two-stage reduction is the preferred method where the inlet pressure is close to or more than 200 psi or where the inlet pressure varies considerably; for example, if the water supply pressure is 250 psi and the desired pressure and flow is 50 psi at 80 gpm. The two-stage pressure reducing station would typically have two pressure reducing valves in series. Each valve would be sized for 80 gpm, with each reducing the pressure 100 psi. Valve number one would reduce the pressure from 250 to 150 psi and valve number 2 would reduce it from 150 to 50 psi. The resulting flow and pressure would be 80 gpm at 50 psi.
Parallel pressure reduction involves the use of two or more smaller-sized pressure reducing valves serving a larger water main or branch. Parallel installations should be used where a wide variation of flow is required for a reduced system pressure and where it is critical to maintain a continuous water supply. Parallel pressure reducing valves provide good pressure control over a wide range of flows. A parallel installation also offers continuous water service if one valve has to be serviced. This allows the bypass valve to be omitted in non-critical installations. The combined capacity of the parallel pressure reducing valves should meet or exceed the maximum flow required for the system.

When using parallel pressure reducing valves, one valve should be set about five to ten pounds higher than the other valve so that the flow will be forced through one valve until the pressure drop is significant enough to cause the second valve to operate. Parallel pressure reducing stations can have two or more valves. A pressure reducing station with two valves can be based on 50 percent flow for each valve or, when off-peak flows are expected to be very low, a 1/3, 2/3 valve arrangement could be specified. The smaller valve is sized to handle 1/3 of the expected flow, and the larger valve is sized to handle 2/3 of the flow. In the 1/3, 2/3 case, an example would be to set the smaller valve to open at 60 psi and the larger valve set to open at 50 - 55 psi.

The smaller valve will operate until the pressure drop causes the larger valve to open.

Combination series and parallel pressure reducing stations are installed where there is considerable pressure reduction and a large variation of flows. These are typically sized with a 1/3, 2/3 valve arrangement when fluctuations in flow are expected. These are more popular in cities with high pressures or where there are large elevation changes in a system.

The reason parallel pressure reducing valves are set at different pressures and sized for the anticipated flows is to prevent the pressure reducing valve from barely opening. When the pressure reducing valve is oversized or when the two valves in parallel are set at the same pressure, the valve will barely open during low flows, causing water to shoot across the valve seat at a high velocity. This usually results in a chattering or whining sound coming from the valve and connected piping system as high velocity water shoots across the valve seat.

This can damage the valve seat when there are impurities like dirt and scale in the water. This condition often leads to a condition known as "wire drawing," where the valve seat wears out at one point from operating with significant pressure differentials and low flows near the shut-off point for long periods of time. If the valve seat is worn or scored at one point, the wire drawing will continue at that point until it becomes difficult to maintan the lower pressure because of valve seat erosion. Wire drawing causes the valve seat to look like someone used a hack saw to cut thin grooves across it. For this reason, many pressure reducing valves have integral strainers built into the body of the strainer to limit the debris at the valve seat. If the valve does not have an integral strainer a strainer should be placed in-line ahead of the pressure reducing valve and between the isolation valves to allow for servicing.

Pressure reducing valve size
Pressure reducing valves should be selected based on the flow and pressure ranges listed in the manufacturer's literature, not on the pipe size that they will be placed in. The selection should be made where the valve is operating in the mid-range of the valve's travel. Having the valve sized properly prevents the valve from chattering or wire drawing. Typically, the pressure reducing valve is one or two pipe sizes smaller than the pipe it is installed in. This generally holds true for larger pipe sizes, but smaller applications with higher flows may require line size valves. Most pressure reducing stations include pipe reducers before and after the pressure reducing valve, because the valve will be smaller than the pipe line size if it is selected properly.

Piping components for a typical pressure reducing station
Following is a sample piping arrangement for a single pressure reducing valve.

1. A main line with tee for a full size by-pass that has a globe valve in the by-pass for manual throttling
2. A ball, butterfly or gate valve in the main line to serve as an isolation valve
3. A pressure gauge with a gauge valve to read the incoming water pressure (gauge to read incoming pressure in mid-range of gauge)
4. A strainer to catch debris that could get caught in the seat of the pressure reducing valve
5. A union to allow for removal of the pressure reducing valve (not needed with a flanged valve)
6. A pipe reducer fitting
7. The pressure reducing valve (sized for the anticipated flow not for the pipe size in which it is being installed.)
8. An increaser (That's a pipe reducer fitting turned backwards.)
9. A union to allow for removal of the pressure reducing valve (not needed with a flanged valve)
10. A pressure gauge with a gauge valve to read the downstream or set pressure of the pressure reducing valve (gauge to read outlet pressure in mid-range of the gauge)
11. An isolation valve (ball, butterfly or Gate type)
12. Then the bypass tee connection
    

Installing the line-sized pressure reducing valve by itself may invite headaches. Engineers should size the valve for the anticipated flow and pressure range. The drawing should have a detail showing the piping arrangement and should specify the associated components that make up the pressure reducing station. Designing and specifying the pressure reducing valve station requires quite a bit more effort than just showing a symbol on the plans.

Ron George specializes in plumbing, piping, fire protection and hvac design. He also provides  plumbing/mechanical code and product standard consulting services and forensic investigations of mechanical system failures.