Valve Authority

So, what is authority? Intuitively, we can imagine a circuit with a pump, a small restriction, and a large valve with which we intend to control the flow. When we throttle the large valve almost nothing happens when it is near the open position, then the flow changes suddenly near the closed position. The valve is said to have poor authority because most of the time something else is controlling the flow.

The problem with low authority in a control valve is that it produces unstable operation during low-load conditions (ASHRAE 2020, 46.9). Thinking about the valve in the situation above, if we intend to set a precise flow rate it might not happen because a small tap to the handle sends the flow above the target and a tap in the other direction sends it below.

The basic definition of authority is (Petitjean 1994, 146):

Some sources use the version (ASHRAE 2020, 46.8):

In hydronic systems, though, the actual variable of interest is the heat transfer of the terminal which saturates with increasing water flow if all else stays the same (Figure 1). This leads to a definition of "Practical Valve Authority" denoted by β':

Here the numerator is the pressure drop of the control valve at nominal flow when fully open, or approximately ((Design Flow)/Cv)2

Figure 1

As a result of terminal saturation, even though the shape of the flow curve might be the same at higher pressure, the fact that it is scaled up means that the shape of the heat transfer curve (heat transfer vs stroke) is not the same at different pressures. As already alluded to, the trouble of low authority shows up as unstable control at low flows. The two curves in Figure 2 have the same resistances within the branch but different differential pressures across the branch. As can be seen, the slope of the higher-pressure curve is about twice that of the lower pressure curve in the neighborhood of the closed position. In Figure 7 we see a comparison for the same pressure as the orange trace in Figure 6, with either no balancing, manual balancing, or a flow limiter. It's important to note that this is the comparison with a pressure that would cause 2x overflow at the fully open position of the control valve without balancing, and thus it's a fairly large pressure being absorbed by the balancing device (for instance if the nominal flow was at 10 psi, this figure would be at 40 psi so the balancing devices would be absorbing 30 psi at the open position). While the curves are not identical, they all have the same maximum slope near the closed position. The curves in Figure 3 all have the same practical authority β' as defined earlier, but different values of β according to the definition that ignores the target flow rate.

Figure 2: Same β, different β'

Figure 3: Same β', different β

Figure 4: After turndown

Clearly from figures 2 and 3, β' is a better indication of authority for a hydronic terminal. In Figure 4 we see the same set of balancing situations as figure 3, but at the pressure represented by the blue curve in Figure 2. Once again, the slopes of the curves all converge in the neighborhood of zero showing that β' is a better indication of authority for a hydronic terminal.

What Pressure Range Should I Choose?

What Does It Mean?

2-32 psi means that as long as the difference in pressure between the two ports on the valve is greater than 2 psi and less than 32 psi, the flow will be within 5% of the rated flow. It's important to note that it's the difference that matters: if the red port is at a gauge pressure of 312 and the blue port is at 300, the flow will be exactly the same as if they were 62 and 50 respectively. The port nearest the inlet should have the red strap and should have the higher pressure.

A Little Background

A flow limiter is a device which normally has a fixed resistance up to some starting pressure, and thereafter changes resistance as needed to maintain a constant flow until a maximum pressure is reached, after which the flow is again like a fixed resistance.

So What Range Should I Choose?

It's important to note that after the maximum regulated pressure the flow goes up proportionally to the square root of the pressure difference. Thus at 10% more pressure the flow will only have increased about 5%, and at 20% more pressure only about 10%. Thus it's normally OK to use the 3-32 pressure range up to around 40 psi. If the pump pressure is over 40 psi, then probably it's still possible to use 2-32 for most of the system, with 5-60 only being used at the terminals nearest to the pump. To actually figure out how near the pump requires knowing the pressure losses in the headers. With variable speed systems using Autoflow, it's common to place the sensor for controlling the pump across the index circuit (the circuit with the least surplus pressure differential), so the pressure difference across any circuit in the building will never go higher than at maximum load. Thus if we take the pressure supplied by the pump and subtract the losses in the main piping until it reaches 40 psi, that's where we would switch from the 5-60 range to the 2-32 range.

Just to clarify about the illustration, it's an over-simplified building showing one terminal per branch. The graph on the left is the pressure difference between the supply and return headers shown as the distance between the green or red line and the black line at any point along the building. The green line represents maximum load, and the red line represents a reduced load with about 40% of maximum for the total flow (perhaps 70% thermal load).

Dual Seal End Pieces


For years, IMI-Flow Design has provided valves with end connections customized for the job at hand.  When field disconnection is desired this means our carefully designed o-ring unions with their superior o-ring retention, but on system connections for an isolation valve a more permanent and tamper-proof system is required.

The dual seal end-piece design is a permanent connection which is far more reliable than other systems.  While allowing IMI-Flow Design to customize valves readily, it provides redundant sealing mechanisms which make a leak nearly impossible.

What It Is

Beginning with the Unibody Accusetter (UA) and Unibody Ball Valve (UB) in 2002, IMI-Flow Design has offered a new style of end-piece.   Since then,  the design has proven highly reliable on more than a million units installed.  It uses a metal to metal seal outboard of the connecting threads, which is almost completely immune to heat.  An anaerobic thread locker keeps the end-piece from being loosened in order to ensure the pressure on this metal-to-metal seal.  Inboard of the connecting threads, where the probability of heat damage is minimized, is an EPDM O-ring.  EPDM was selected due to its excellent performance in aqueous environments and its excellent resistance to compression set.

Exclusive Dual Seal Endpiece
Exclusive Dual Seal Endpiece

Also visible in the figure is the retaining nut for the ball valve.  By setting the compression on the ball valve seats separately from the torque on the end-piece, IMI-Flow Design is able to ensure that each is appropriate for its purpose: other designs can potentially require a compromise to seat tension or worse still inadequate torque on the body's sealing surfaces.

Bottom Line

IMI Flow Design can provide valves with the right ends to prevent the need for field-installed adapters.  These ends will be installed at the factory and include exclusive and extremely reliable sealing systems.  The net effect is less effort and fewer worries for the installer.

Why’s There A Nut Under That Handle?


Many products from IMI-Flow Design have an integrated ball valve for isolating the terminal.  Under the handle of these ball valves there's a nut.  This is called the "packing nut".



Concealed under the packing nut is... wait for it... packing.  Specifically, there's a piece of PTFE which is squeezed by the nut to form a seal against the body and  stem.


Cross Section showing stem seals
Cross Section showing stem seals

Here the packing is shown un-deformed, overlapping the bottom portion of the packing nut.  In the actual valve it re-shapes itself and flows as the nut is tightened.


O-rings are great.  As can be seen in the cross-section, we use one of them on our ball valve stem.  Unfortunately all elastomers have an Achilles heel: it's always possible for some combination of temperature and chemistry to make them fail.  PTFE is very resistant to heat and chemicals, but it isn't "rubbery".  Thus the PTFE makes it possible to stop any stem leak, but the nut must be tightened to squeeze it against the surfaces.  If the PTFE gets worn from turning the handle many times, the nut can simply be tightened a little more.


It's important to note in the cross section above that the stem on IMI-Flow Design ball valves is "blowout proof".  That is, there's no way that someone can loosen something and have the stem come out.  This is an important safety feature which is widely accepted, but some of our competition has introduced valves lacking this feature.

The end connection design can also be seen.  Again an o-ring is used to provide the primary seal.  Again there is a secondary seal: in this case it is metal to metal.  This end connection allows IMI-Flow Design to customize a valve so that both ends match their mating devices directly with no adapters.  The dual seals on this end connection ensure that it will not leak.

Three Classes of Flow Controller

Class I

These are the simplest flow controllers mechanically.  The differential pressure across the device acts on an area, which tends to move a piston.  This force is opposed by a force which varies with position (normally a spring).  The force due to pressure and the spring force determine the position of the piston.  The piston is arranged next to some sort of seat, so that between them they form an opening: that opening is precisely tailored so that it gives the correct Cv for the target flow at the corresponding pressure.

Advantages Disadvantages
Full DP drives piston Not Adjustable
No Seals Required Porting Design Requires Expertise

Class I example

In the graph below, the trace "fo" shows the flow through fixed orifice, the trace "vo" shows the flow through the variable orifice, and then there is a trace adding them together to show the overall effect.  In the case of the version shown above, the fixed orifice would be the flow around the piston plus the hole in the end of it.  The variable orifice is the contoured shape in the side of the piston.

Flow components through Type I regulator

Class II

Class II flow controllers have a simple, direct acting differential pressure controller keeping a constant pressure across a control restriction.  In other words, these have a control restriction which sets the flow rate, and a dynamic restriction which changes to maintain a constant pressure across the control restriction.  The piston, in this case, is driven by the set-point differential pressure.  In other words, a class II flow controller with a starting pressure of 2 psi would always be moved by 2 psi.

Any disturbance force directly translates to an error in this pressure, so disturbance forces must be kept very low compared to the active area of the piston.  These disturbance forces can be created by the flow of water past the dynamic restriction, by friction, or even by gravity.  A simple class 2 regulator is shown below.  In this case, the fixed restriction is formed directly on the piston, although more advanced design have it separate.  The dynamic seal is needed to achieve accuracy because leakage cannot be compensated by porting as it can in Class I.  This is a challenge since friction from this seal will create hysteresis.

Class 2

Advantages Disadvantages
Adjustable Only starting pressure drives piston
Easy to make changes to flow Seals required to achieve accuracy
Bulky compared to flow

Class II are good for small flows where size is not such an issue, and where adjustment is required.

 Class III

Class III regulators control differential pressure across a restriction much like class II.  The difference is that class III use a pilot mechanism to control the motion of a larger restricting mechanism.  As a result, they can be much more accurate for a given set-point.  For instance, Equalizer (patent applied for) is a Class III regulator.  A pilot mechanism sends either P1 or P3 to the main mechanism, based on the controlled pressure difference (P1-P2).  The result is that as long as the pressure difference is enough to move the main mechanism, the accuracy is only a matter of how accurate the pilot device is.  In class II, any friction would directly create an error based on how it compares with the controlled pressure differential.  The result is a device with the adjustibility of Class II, but with robustness similar to Class I.




The question will naturally arise how I can refer to a pressure controller as a flow controller.  In the end, though, that's exactly how they are used in these systems.  The difference is, a pressure controller is a flow controller where the control orifice is created by the user.  The flow is determined by the setting of the pressure, but dynamically it depends more on the resistance of the load.  That load can be a control valve, a local circuit, or a branch in a building.



Venturi Balancing Valves

IMI Flow Design sells manual balancing valves based on a fixed venturi element.  This brings up certain questions, of course.

What's a Venturi?

A venturi, at least in this context, is a device which converts pressure to kinetic energy, then converts it back.

To put it simply, it gets narrow, then widens out gradually so as not to stir the water up too much.  As the passage narrows, the pressure goes down.  That might sound confusing, but the fluid has to speed up and it only does that when there's more pressure behind it than in front of it.  As the passage gradually widens, the pressure increases again.  That might also seem confusing, but the fluid will be slowed down by some combination of friction with the wall and increasing pressure: if we avoid too much stirring and make the passage smooth, most of the slowing down will be from increasing pressure.

For a flow meter, we have a connection to the water stream before the passage narrows, and a second port at the narrowest point which we sometimes call the "throat".  The pressure at the throat is lower than the downstream pressure, so the pressure difference we read is higher than the permanent loss that's created.  That last sentence is the whole point.  Figure 1 shows how the pressure changes as water flows through a venturi.  On top is a graph of pressure, and below is a cross-section showing streamlines.

Venturi Flow
Figure 1

Why Use a Venturi?

  1. Because it conditions the flow, a venturi is less sensitive to upstream and downstream conditions than a simple orifice.
  2. The signal pressure on a venturi is higher than the permanent loss.  Whatever signal the balancing company considers acceptable, the pumping loss will be lower with a venturi than with a non-recovering flow restriction.
  3. IMI Flow Design's venturis have been tested to ensure that at any given flow rate the pressure signal is the same regardless where the balancing valve handle is set: so just find the target pressure and dial it in.
  4. IMI Flow Design's venturis are accurate within 3% or better.  This kind of accuracy comes from a simple geometry made with CNC machinery.  Most other kinds of differential pressure flow meters are less accurate because they depend on too many variables.


Modulating Control Valves and Autoflow™

The question comes up regularly: does Autoflow™ interfere with the operation of a modulating control valve? After all, as the control valve closes, the Autoflow will be opening.

In short, of course it will make a difference, but it will not be worse than without the Autoflow™.


The rate of heat flux from the water in a coil to the air depends on the temperature difference between the two at the point of contact. Thinking about this, it's clear that the theoretical upper limit of output for a coil would happen when the inlet water temperature was the temperature at the inner surface of the tube over the whole surface. Thus, as flow increases, the heat transfer "saturates" as the temperature of the water leaving the coil gets closer to the temperature entering.

Heat flux saturating
Figure 1

It is not uncommon to design a terminal so that the output under design conditions is 95% saturated with respect to water flow. Under these conditions, doubling the water flow would only make a 4% change in output.

Square Law

Water flow is typically turbulent: It's just a consequence of high density and low viscosity. The result is that when the flow of water through a fixed restriction is reduced to 50%, the pressure drop becomes about 25%.

This is significant, because as the control valve restricts the flow, all other pressure drops in the local circuit become trivial and the control valve takes all the pressure drop. If a balancing restriction of 32 psi is made by a manual valve, at 50% flow it will be making an 8 psi pressure drop. For Autoflow, the pressure drop will admittedly be down to about 0.25 psi. The thing is, though, that the control valve is now seeing an extra 24 psi in one case and an extra 32 psi in the other. Assuming it was sized for 5 psi, this means comparing 29 psi to 37 psi. The square law works the other way when comparing flow at different pressures: the Autoflow system would flow about 63% of maximum if the manual valve is at 50% of maximum.

Net Effect

With a properly characterized control valve, the heat transfer as a function of stem position is almost exactly the same whether manual balancing, automatic balancing, or no balancing is used:

Flux with Autoflow, manual, or nothing
Figure 2

This might prompt the question why to balance. The answer lies in comparison of the water flow under the same conditions:

The horrors of overflow
Figure 3

So, while the thermal result as a function of stem position is almost the same, the unbalanced system will waste water flow ridiculously when the demand is near to the design conditions. So if the water flow is so much higher, why is the heat transfer not? Simple. The "delta T" of the water is very low for the unbalanced system.


Even in the case when Autoflow™ must absorb 32 psi to prevent overflow, it does not significantly affect the performance of the control valve.  This is because regardless of how the system is balanced, the control valve will see very nearly the full pressure drop across the circuit by the time it throttles to half flow, which is typically 80% of heat transfer, and overflow contributes very little added heat transfer.

Thus, a commonly cited argument against automatic balancing is a misconception.  The advantages, however, are not.  An automatically balanced system prevents wasted circulation under all conditions while allowing variable speed pumps to ramp back without starving the system.  Automatic balancing also allows the system to work correctly from the moment it starts up, without a complex adjustment procedure.  Finally, the use of fixed flow limiting cartridges such as Autoflow™ prevents uninformed personnel from unbalancing the system.

Traction Control

Picture yourself on a soggy dirt road. For some reason you've stopped and want to get going again. Unfortunately, when you press the gas, one wheel spins. Pressing the gas more just makes it spin more and the car goes nowhere. Then you suddenly remember your car has traction control and activate it. The system applies the brakes to the slipping wheel so that power can go where it will do the most good.

Some say that adding balancing devices is like driving with the brakes on. In a way they're right: balancing valves can only make the water flow slower in the circuit they're connected to. Below we'll explain why one would want to do that.

How Can I Have Spinning Tires in a Hydronic System?

The output of a coil "saturates" as the flow of water through it increases. That will be explained later for those interested. The point is: beyond a certain point more water flow helps very little in cooling or heating the room. Too much is pretty much like a spinning tire. If there's twice the design flow, there might be only 5% more heat transfer while another room is being robbed of capacity.

Hydronic Traction Control

As with traction control on a vehicle, we slow the flow through the wild circuit so that power can go where it will do more good. Autoflow™, in particular, is good as traction control. If overflow would otherwise happen, Autoflow™ will add just the right restriction to prevent a large waste of flow. When the pump slows down or the control valve starts throttling so that an overflow would not happen, Autoflow™ opens fully so that it creates very little restriction. As the flow continues to reduce, the pressure drop of the balancing device reduces according to the square law, so at 50% flow the pressure drop through the balancing device is ¼ of what it is at full flow.

Why Does a Coil's Output Saturate?

Heat transfer is dictated by the temperature gradient and some coefficients. This means that for a given incoming air temperature, air flow, and water inlet temperature there's a theoretical limit for the heat transfer. If we imagine that the inner surface of the coil is all at the inlet water temperature, clearly the coil could not transfer more heat than what we would calculate. Also clearly, that state will never be reached: as we transfer heat the water changes temperature. For a typical air conditioning coil, the design flow is often as much as 94% saturated meaning that it produces 94% of that impossible theoretical limiting heat flux.



So, if a terminal flows twice what it should, the heat flux increases less than 6%. This remains true if it flows 4 times design flow. All that extra flow does is make noise, waste pump power, and make the chillers less efficient.