What makes a good PICV?

[edit] Introduction

Over the last five years, PICVs (Pressure Independent Control Valves) have been widely accepted as the best method of terminal control in variable flow systems due to their energy saving potential. The surge in popularity has led to an influx of products with varying designs, features and functionality. This article reviews some of the mechanical PICV design elements and how they can impact on the PICV’s performance in an applicational context.

[edit] Where do we start?

To help specifiers and project engineers assess which PICV is best suited for an application, the BSRIA BTS1/2019 standard has been developed to provide a consistent test method for PICV manufacturer’s products to be benchmarked against.

Manufacturers should be able to provide test results in line with this technical standard which covers:

• Measured flow vs nominal flow.
• Pressure independency or flow limitation.
• Control characteristics, both linear and equal percentage.
• Seat leakage test.

Repeatability and accuracy are central to the tests, and they are key to good temperature control and realising the full energy saving potential of a PICV installation. An accurate PICV means the measured results will be equal or very close to the manufacturer’s published nominal flow rate each time it is measured; this is known as low hysteresis.

Accuracy has a positive impact on a building’s energy consumption. “Measured over time, a 1% increase in the accuracy of a PICV can result in a reduction of around 0.5% in the building’s overall hydronic energy consumption” (FlowCon International).

Valve accuracy is driven by the design, manufacturing process and material used for the internals of the valve.

• The design of the PICV should allow for full stroke modulating control at all flow settings without any stroke limitation. The flow setting and temperature control components should operate independently. Some PICV designs use the stroke of the actuator stem to set the flow rate resulting in limited stroke and control. This can cause issues at low flow rates whereby the PICV effectively becomes on/off irrespective of actuator selection.
• The manufacturing process and the component materials also contribute to accuracy. For example, injection-moulded, glass-reinforced composite materials cope better with water conditions that valves can be exposed to. They also have less material shrinkage than other materials, delivering higher accuracy than valves that use alloy components.

[edit] What else should be considered?

The importance of accuracy and repeatability are paramount when selecting a PICV. However there are other factors that should be considered:

• Wide flow rate range – including low flow rates for heating applications, ideally covered by a small number of valves.
• Setting the flow rate – setting the PICV can influence the accuracy. There are various scales used including set points related to flow rates and percentages. PICVs with very detailed scales with small increments between set points are more difficult to set accurately, leading to higher tolerances than the BSRIA standard recommended + 10%.
• Wide ΔP Range – low start up pressure. To operate satisfactorily, the PICV requires a minimum pressure differential to overcome the initial spring resistance within the PICV, enabling the spring to move and take control. Care should be taken to ensure the minimum pressure differential is as low as possible to maximise the energy saving potential of the system. The maximum DP should also be considered to ensure the PICV operates effectively under part load conditions.
• Dirt tolerance – the valve control opening area on all PICVs, irrespective of the manufacturer, is identical for each flow rate. The shape of the control area can be different depending on the valve design. A rectangular flow aperture is more tolerant than an annular flow aperture. Debris will pass through the rectangular aperture more easily.
• Removable inserts – deliver the greatest flexibility and serviceability. Products can be easily serviced in line without disruption. This is especially of value when water quality is poor or when flow requirements change due to changes in space usage. Inserts can also be removed during flushing. Valve bodies can be installed with blank caps eliminating the risk of damaging or contaminating the PICV element, whilst having a full-bore flushing capacity.
• Installation – PICVs in general have no installation restrictions. However, in line with BSRIA BG29/20, it is recommended that PICVs should be installed in the return branch as small bore PICVs will have a high resistance which will hinder the flushing velocity during the forward flushing of terminal units.

[edit] Making the right choice

There are many aspects for specifiers and project engineers to consider when selecting the right PICV for an application. The BTS1/2019 standard provides an excellent benchmark, but the individual designs also need to be carefully considered. A correctly selected PICV will ultimately lead to a more comfortable indoor climate with better control of the space heating and cooling as well as potentially reducing the pump energy consumption in a building by up to 35%.

This articial originally appeared on the BSRIA blog website. It was written by Andrew Pender, National Sales Manager at FloControl Ltd and all views expressed are those of the author. It was published on 22 September 2020.

--BSRIA

[edit] Related articles on Designing Buildings Wiki

• BSRIA articles on Designing Buildings Wiki.
• BSRIA launches updated Valve Test Standard.
• BSRIA publishes new edition of BG29.
• European hydronic controls market.
• Differential pressure control valve.
• Flow characteristics.
• Pressure independent control valves.
• Valves.

[edit] External resources

• FlowCon International, Composite Perks - Why use Composite in PICVs?