The MBV operates with a fixed, although manually adjustable, resistance. The flow through the MBV depends upon the degree to which it is throttled and the differential pressure placed across it.
MBVs can be subdivided into two types depending upon their method of measuring flow. There are fixed-orifice-type and variable-orifice MBVs.
The fixed-orifice MBV (FOMBV) separates the throttling and measuring functions. Typically, the throttling is performed with a ball valve (although the British prefer a globe valve) and the measuring is accomplished with either a venturi or an orifice plate.
The two components are generally combined into the same casting, or the devices are screwed together to enable the required throttling and measuring functions.
The variable-orifice MBV (VOMBV) measures the pressure drop across the throttling valve. As the VOMBV-style balancing valve is throttled, the flow is determined by measuring the pressure drop across the resistance formed between the valve seat and plug (assuming a globe-style valve).
There are three significant advantages that the VOMBV has over the FOMBV, and two have to do with accuracy:
1. As the VOMBV is throttled to reduce flow, the orifice formed between the precision valve plug and seat becomes smaller (lower flows are measured across a smaller, more accurate, orifice).
2. As the VOMBV is throttled to reduce flow, the measured pressure drop across the plug and seat (flow signal) increases (lower flows produced by more throttling result in a bigger, accurately measured, pressure drop-flow signal).
3. When a VOMBV is shut, the measured pressure drop is equal to the available pump head (if the piping is open). When the throttling valve of an FOMBV is closed, the measured pressure drop (flow signal) is zero.
It is very easy for the VOMBV to identify available pump head, partially or completely plugged pipes, and reverse and extremely low flows.
As the water attempts to flow faster through the slotted cup, the increased pressure drop across the cup pushes it back into the sleeve and therefore exposes a smaller area of the slot through which the water flows.
The flow through the FL is constant regardless of the pressure drop placed across the device, as long as the slotted cup is free to move. Even though the pressure drop across the device can be measured, the position of the cup and the amount of slot area open to flow is unknown, so no flow measurement is possible.
The flow limiter is therefore not considered to be a balancing valve, because it lacks the ability to measure flow.
The major differences between the MBV and the FL lie in the ability of the MBV to measure flow as the differential pressure changes across its measuring section, and the ability of the FL to produce a constant flow by absorbing various amounts of pressure drop.
A hydronic system without MBVs achieves flow based upon the resistance of the associated pipe and fittings, rather than by load requirements; the general tendency is to overpump portions of the unbalanced system without having any way to determine what the actual flows are, or a means to adjust the flows through any particular piece of equipment.
The MBV is merely throttled to trim the flows to their design limits and to verify these flows. No additional pressure drop is imposed on the hydronic system, as the present batch of MBVs doubles as shut-off valves.
As long as the system supply-return pressures remain constant, the MBV has the edge in features because it can verify flows. When the system pressures vary, the FL may appear better, as long as the terminal unit has a constant flow requirement.
The problem occurs when an FL is used in conjunction with a two-way modulating control valve. The scenario runs like this: All is well as long as the modulating two-way control valve is at design conditions (wide open) — the FL absorbs any excess pressure drop so that the flow through the terminal device is held to maximum design levels.
Unfortunately, the control valve does not spend most of its time in this state; as the space comes under control, the controller attempts to reduce flow by gradually closing the two-way control valve.
The FL, on the other hand, “senses” the attempt to reduce the flow and merely opens to maintain the flow at the FL (maximum design) set value. The control valve fights in vain with the FL to reduce the flow until the FL runs out of the spring range.
All this time, the controller is getting no response from the control valve’s modulation; a portion of the control valve’s stroke (originally assigned to handle off-design loads) has been wasted with no result. The control loop becomes stressed and may even fail, causing hunting with the resulting discomfort, plus premature component failure.
Since three-way control valves are designed for constant flow through their “common” port, the FL has no advantage and the owner loses the flow measuring capability. Flow limiters can most often be found in systems with varying system supply to return pressure drops, but constant equipment load-flow requirements.
It has been shown that the “hand temperature” or even pocket thermometer measuring methods produce poor temperature vs. flow correlation, particularly in excess flow conditions.
It is important to bear in mind that a pump was not designed to be a rotary flow meter and that coil pressure drop data are created to ensure that the coils have the capacity to transfer the indicated amount of heat without absorbing more than the indicated pressure drop (it is not designed as a flow-measuring tube).
Balancing valves, and in particular VOMBVs, provide the tools to diagnose a multitude of hydronic system anomalies:
1. Close the VOMBV; the pressure drop you read across the valve is the supply-to-return differential pressure which is available to drive the water through the associated coil. (Is there sufficient supply-return pressure available to meet design requirements?)
2. Closing the VOMBV with no rise in observed pressure drop means that either some other component in the circuit is closed, or the piping is plugged.
3. Closing the VOMBV and observing a slowly increasing supply-return pressure is an indication of either a non-bubble, tight-closed valve or an almost-plugged pipe, strainer, etc.
4. If the pressure drop across an open VOMBV is too low to measure, gradually close the valve to increase the pressure drop which enables measurement, and thereby helps determine the cause of the very low flow.
5. Higher pressure on the outlet measuring port of the VOMBV than is seen on the inlet port to the valve indicates that a flow reversal has occurred. (Flow reversals, often transient in nature, can appear or disappear as the loads change in other parts of the system.)
Since the MBV throttles and measures flow, be sure that the manufacturer publishes the accuracy at all valve positions and flows; there is no other way to compare the accuracy of an MBV, and accuracy is a major portion of what is being purchased!
Sizing of the MBV is also important. Just like their control valve counterparts, balancing valves should be sized for their maximum flow requirements, rather than by pipe connection size. These guidelines are available from the manufacturer.
Here is where the VOMBV shines again. An FOMBV, with its fixed-flow measuring orifice size, creates a significant pressure indication at maximum flow, but must use this same orifice for lower flow measurements as well.
On the other hand, a larger VOMBV with its plug almost closed has the same throttling-measuring characteristics as a wide-open but smaller connection size valve. It is therefore easy to use a larger valve to effectively measure very low flows.
The penalty for oversizing a VOMBV is that bigger valves cost more, and that VOMBVs are slightly more accurate when positioned closer to their wide-open position.
The correct selection of balancing valves can result in an efficient, well-functioning hydronic system while providing a means to diagnose and modify flows when things don’t happen as expected.