TOTAL VALVE SOLUTIONS

Pressure reducing systems for housing complexes

Housing complexes need pressure reducing valves for several reasons:

  • Higher supply pressure (from municipalities and waterboards) to meet growing demands.
  • Lower required pressures to homes to reduce any potential leaks.

The solution is simple and inexpensive for traditional free-standing houses. A line size (40mm, 32mm) direct acting pressure reducing valve costs less than R1500.

It’s more complicated in housing complexes where water to individual homes comes from a common supply.

Ineffective pressure reducing solutions often lead to catastrophic results: broken pipes, huge water losses and homes without water for extended periods.

This article provides guidelines for the correct selection and installation of pressure reducing systems for housing complexes.

There are 3 types of pressure reducing valve:

  1. Direct acting valve. A very simple device with an adjustable handle to set the pressure to the required value.
  2. Pilot operated valve. Usually used for large systems and often the only solution for housing complexes.
  3. Ratio reducing valve. Reduces pressure in a ratio between upstream and downstream pressure.

The direct acting valve.

These are limited in size range and capacity. Often a 100mm direct acting PRV will be too small to handle the required capacity for a typical 100mm pipe feeding a housing complex.

The pilot operated valve.

This is the most commonly used valve for housing complexes as they have a much larger capacity.

Issues to be aware of with these valves are as follows:

Sizing. These valves have to be carefully sized to ensure that they operate within the design flow rate because they can become unstable at higher or lower flow rates leading to wild fluctuations in pressure and the possibility of broken pipes.

Minimum and maximum inlet and downstream pressures must be considered to ensure that the valve is stable under both extremes.

The biggest problem is the minimum flow rate (which occurs at night). The solution is to install a small direct acting PRV as a bypass to the main valve. This bypass valve will be set at approx. 1/2bar higher than the main valve. At night the main valve will automatically close and all the low flow will then go through the bypass PRV.

Dead–end service. The valve must maintain the set downstream pressure under zero flow demand conditions eg in the middle of the night.

Pilot operated control valves take time to react. The valve might not be able to react in time if the demand ceases suddenly. This leads to an increase in downstream pressure and the risk of broken pipes.

The solution is to install a relief valve immediately downstream of the PRV to blow off for short periods during demand closure.

The ratio reducing valve.

This valve has the great benefit of instant reaction time and also being able to handle very low flows which does away with the need for a small bypass PRV.

It is the most simple PRV and is becoming more popular as users discover its enormous benefits.

Its only disadvantage is that it is not adjustable. This is not always a disadvantage because it means it is tamperproof. Tampering is a significant cause of incorrect operation and expensive problems.

All pressure reducing installations should include:

  • A strainer upstream. Foreign objects often get into potable water piping due to pipe breaks and other causes. A strainer will protect the installation from these.
  • Isolation valves either side of the installation to ease repair and maintenance without interrupting supply for too long.

Reputable control valve suppliers will assist with sizing and usually provide all the components made up as a single unit. The supplier should be involved in commissioning after installation.

New Developments in the Air Valve Evolution

Air vales have received a lot of attention because they play such a crucial role in pipelines. A brief summary of some of these developments follows:

Standard air valve. Normally produced with a reduced bore to make the body smaller and thus less expensive. These valves are prone to close under high air velocity (pre-closure) if a pipeline is filled too quickly, making them inoperative.

Kinetic air valve. These are equipped with a kinetic shield under the float to prevent closure under high air velocities. The problem with this approach is that the when float closes as the water arrives it results in a high local “slam” often causing damage to the pipe or valve.

Non-slam air valve. Initially developed to protect the air valve and eventually used for surge suppression. Early versions had a separate restricted check valve under the air valve to slow approaching air and water. This became so popular that it was incorporated into the valve by most manufacturers.

A proliferation of these valves appeared on the South African market after the SA patent for the non-slam design ran out.

One of the problems is that these valves cannot reduce the speed of the water column when all the air is already out of the pipeline.

The hope that this design would finally solve all water hammer problems associated with air valves has not been realised. Even with the correct switching point there are certain scenarios where the Non-Slam air valve does not produce the required results.

Dynamic air valve. The dynamic air valve is the latest development from the A.R.I. valve works. It solves many water hammer problems that the standard non-slam valves don’t.

The valve reduces velocity (and thus water hammer) by allowing water through the valve for about a second after all the air has been expelled.

This valve has shown significant benefits during independent testing by Mekorot (the Israeli National Water Company).

The results of the testing were published in the Journal of Applied Fluid Transients in an article by Yiftach Brunner of A.R.I. and Sathish Kumar of Fluidhammer Consultancy Services.

The full report is available here.

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