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Issues of back flushing PIC valves explained

Author : Martin Lowe

07 July 2017

The popularity of Pressure Independent Control (PIC) valves over the last few years has significantly increased and now are the popular choice of many system designers.

The initial problems years, introducing PIC valves have gone and the basic understanding of how they work, what components they replace, the best commissioning method to achieve repeatable readings and their problems are understood.

It might not come, as any surprise, that with any moving part completely submersed in water for its entire working life that water quality is of prime importance.

Those who are familiar with water balancing of heating and cooling distribution systems understand the importance of good clean water.

The long history of flushing pipework, back flushing terminals and keeping the water clear of those inhibiter loving bacteria that can cripple a water distribution system within days have been the concern of the industry for years.

Over those years and many installations since the first PICV’s were installed, what has been learnt?

Is there still a problem that is still waiting that new innovative idea, that change in practice that will put everything right, that product enhancement that makes us all sleep better at night?

It’s good to know that the traditional dirt is still a problem today as it was when balancing was first introduced. In fact, tradition might be the cause of some of the new problems that have appeared in systems using PICV valves.

When back flushing was first introduced on manual balance systems using two port and four control valves, the few moving parts were always opened fully before any back flushing took place.

Any dirt or debris was carried past the DRV balancing valve, then through the fixed orifice, then the control valve seat, and finally through the terminal device and out into a bucket or drain.

The balancing valve had been positioned in the return because of its design and its ability to act as a ‘shut off’, the orifice plate was often close coupled to the DRV and required at least five diameters of straight pipework up stream to allow accurate flow measurement.

The two port control valve and the three port control valve were positioned in the return because they were the least extreme in heating or cooling. De-airating was not common, and a drop in pressure before a terminal coil heat exchanger that would be seriously affected with the release of air, could easily be avoided by putting that piece of equipment after the terminal device.

So tradition, with good reasoning, moved forward and control valve and balancing valve were conventionally place in the return.

On the arrival of the PICV and as a direct replacement for the two port control valve and the balancing valve, the return pipe work looked the ideal location. As pipework design is, to a degree, based on what worked last, schematics started to show the PICV valve in the return.

Even the BSRIA Energy Efficiency of Pump Systems Guide shows the PICV valve in the return. What was not understood at the time, and is the point of this reflection, is that many PICVs should not be back flushed through.

Some manufacturers actually state that back flushing is not recommended and voids the warranty. Why is back flushing not recommended through a PICV? The answer is dirt and delubrication of the moving parts.

The problem with back flushing through a dynamic device is that flow rate is not control. When the return isolation valve is opened the amount of pressure behind the water flushing through the valve is dependent on a number of factors.

Those factors are related to the individual valve positioned in the system, if the pump is running and what other floors and valves are isolated.

This increased in flow velocity, flowing in a direction not designed by the manufacturer with water quality that is always suspect, could possibly damage the diaphragm, closing the shutter restricting the flow, that further increases the pressure on the diaphragm.

How can you tell when a PICV has been damaged? Back flushing is facilitated by having a drain before the PICV. The inclusion of any drain, is an invitation to flush, as flushing has been traditionally an approach to removing blockages on static systems.

What is now being found is that that the drain before the PICV is causing damage to the internal workings and effecting the cartridge performance after flushing.

Typically, it is seen that the increasing pressure curve moves higher than the design flow and the hysteresis on the downward curve increases.

The easiest method of preventing back flushing is to place the drain after the PICV, which usually means putting it in the flow. This move against tradition can be accommodated by the fact that de-aeration equipment is used on every project and it works.

The differential pressure is split between two seats, the first seat prior to the control valve seat.

Ensuring the water quality before the PICV valve is still very important and a close coupled flushing by-pass properly used will significantly increase the water quality.

Many manufacturers might claim that their valve is suitable for back flushing, but this is not measurable and what could be acceptable on one valve in the system could be unacceptable for another.

There is no way of knowing or measuring the amount and length of back flushing.

It is actually easier at the design stage to move the PICV to the flow and then insert a drain between the valve and terminal unit.


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