Why care about Valve Authority?

​Quality, Consequences and the Construction Industrial Complex (part 44) – All IMHO:

How difficult can it be to comment on control valve authority plus CHW & HTG coil standard details? It should have been an easy task, but it was not at all.

Researching this post has both saddened and enraged me a little bit. Lets get real, making people in buildings comfortable is not life and death. Yet doing this post has reminded me how little consensus there is in the MEP & HVAC design and construction community and importantly, how few consequences there are for poor work.

So what got me going? I wanted to highlight the importance of control and Commissioning valve authority plus provide an example of a generally agreed CHW & HTG coil pipework detail from a Commissioning and Facilities Management perspective. Easy right? 

Lets start with the importance of control valve authority. When I see a line size control valve in a HTG or CHW installation I am immediately suspicious that the valve has not been sized for authority. In my experience there are many times (not always) when the control valve needs to be one size less than the line pipe size to obtain some authority. 

Many design engineers I spoke to, do not believe sizing the control valves for authority is their job! IMHO it absolutely is.  

Quality is a million little things, that when done well, provides a cumulative effect that is greater than the sum of the parts. In effect, 2+2=5. Control valves are very important. Control valve authority matters because:  

• Valve authority is the basis for control and commissioning valve selection.
• If the control valve is over sized it will effectively function as an on/off valve rather than provide some form of proportional control.
• If the control valve is over sized it will effect coil performance plus system efficiency and energy performance.
• If enough control valves are over sized, the performance and life span of the chillers will be impacted.

This is one reason amongst many, that energy modelling (a mix of science and art) frequently fails to accurately predict building energy performance. In an energy model, there is an incorrect, IMHO, basic assumption of correct system design, operation and obtained efficiencies. If the controls valves are not sized correctly there is an immediate disconnect between the energy model and real world performance. Also, sadly, nobody gets sued for over sizing, only undersizing.

So what to do? I do not want to reinvent the wheel here so I will point you to an excellent paper on “Control Valve Theory and Selection” by Robert Bean at “www.healthyheating.com” which covers valve authority in detail. However the main points IMHO are:
 

• To obtain some control it is recommended that the control valve size is selected so that its authority is never less than 0.5.
• For clarity: valve authority between 0 – 0.25 = unstable control; 0.25 – 0.5 = fair to good control & 0.5 – 1.0 = excellent control. 
• The pressure drop through the control valve should be equal or as close as possible to the pressure drop through that part of the system or equipment in which the flow is varying.
• Install the Control valve with optimum inlet and outlet conditions i.e. minimum 10 pipe diameters in and 5 out. 
• Install the control valve in the return pipe, YES, the return pipe. On this point, there is a North American Vs the rest of the world difference of opinion!

Control valves are frequently installed in the coil flow pipe in North America, but this can provide a higher pressure drop though the coil. So why install the control valve in the return pipe of the coil? What are the benefits?
 

• With the control valves installed in the return, the overall pressure drop, pump head and noise generation can be minimized;
• the coil will be fully flooded in the event the control valve is shut;
• there will be stability of flow through the coil; 
• there is a reduction of, off gases, thereby lowering dissolved oxygen content. Lower dissolved oxygen extends equipment life by minimizing air entrapment;
• for HTG, some manufacturers have temperature limitations on control actuators, the lower temperature return location is preferred for these actuator types; 
• for CHW it is preferred to have less thermal breaks on the flow into the coil;
• CHW return side control valve installation reduces sweating on the electronic actuator and extends the life of the valve seat. 

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This post is getting too long so I will provide the HTG and CHW coil detail (with notes) optimized for Commissioning and facilities management next week. In the meantime I expect this post to generate some haters. Why? Because there is little agreement on good practice and very little consequences for poor copy and paste work. When will the building design and construction community care about details like this? I know this is an unfair generalization but I see these problems on many projects so writing this, I have a sad face ? …

I strongly recommend the sources listed below, as they represent best practice IMHO.

Sources:

• Robert Bean, www.healthyheating.com, “Control Valve Theory and Selection”
• Robert Petitijean, Tour & Andersson, “Total Hydronic Balancing”, the ultimate, go to book on Hydronic systems
• Hailey Mick, TA Hydronic College. “Control Valve Authority & Pressure Independent Systems
• Martin Friedman, retired Engineer and font of all Hydronic systems design wisdom