some application on stap

8/20/2019 Some Application on STAP

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Some applications in HVAC of self-acting differential pressure controllers

To enable stable and accurate control, differential pressures across modulating control valves should notvary too much. This can be obtained with a self-acting differential pressure control valve STAP in avariable flow distribution. This solution has some specific advantages:

1- Enable stable and accurate modulating control,2-

Minimise noise from control valves,3-

Avoid interactivity between circuits, which are maintained independent.

1- How it works

Fig 1: STAP stabilises the secondary differential pressure DpL.

STAP is a self-acting proportional control valve that stabilises the supply differential pressure of a circuitor a branch with several circuits.

The design of STAP is based on a spring-membrane combination. The spring pulls the balanced plug(2) to open the valve. Differential pressure AB is applied on the membrane (3), creating a force againstthe spring. Pressure A is communicated to the STAP by means of a capillary connected to the drain (6) ofthe measuring valve STAD/M (STAD or STAM). Pressure B is communicated internally to the other sideof the membrane.

The measuring valve may be cancelled and replaced by just a test point on the pipe. This is notrecommended if water flow is not measurable using another feature.

When the force created by the differential pressure AB on the membrane is higher than the force of thespring, the valve starts to shut proportionally until it finds a new equilibrium position. This creates asupplementary pressure drop in the STAP that limits the increase of differential pressure DpL.

The design force of the spring is modified with an Allen key introduced through the centre of thehandwheel (7). This allows the adjustment of differential pressure DpL to the required value. Thehandwheel (7) itself is used to shut the STAP to isolate the circuit when necessary.

Water flow can be measured easily with the STAD/M and a balancing instrument “CBI”. SecondaryDpL is measurable between (5) and (4b), (when 4b is equipped with an optional test point) or between (5)and (8).

As STAP is a proportional controller, differential pressure DpL is not maintained absolutely constant, but



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2- Some applications in air conditioning

2.1 One STAP on each riser

STAPSTAD/M

Fig 4: A p controller STAP stabilises the differential pressure on each riser.

For large systems, the pump head may be too high or variable for some terminals. In this case, differentialpressure is stabilised at the bottom of each riser, at a suitable value, with a STAP differential pressurecontrol valve.

Note: In heating applications, if all the control valves of one riser shut, the differential control valveSTAP will also shut. All the return piping of this riser decreases thus in static pressure as the water coolsdown in a closed area. Differential pressure across the control valves becomes much higher. As a

consequence, the control valve that reopens first will be temporarily very noisy. A minimum flow createdby a relief valve BPV, for instance, avoids such a problem.

2.2 One STAP on each branch

STAP

STAD/M

Fig 5: A p controller STAP stabilises the differential pressure on each branch.



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Technically, this solution is better than one STAP for each riser, because the differential pressurerequired on each branch may vary. Moreover changes in differential pressures due to the variable pressuredrops in pipes of the risers are automatically compensated.

Regarding the minimum flow, note given under section 2.1 for risers may be extended to branches.

ExamplesIn figure 6a, each terminal unit C is provided with a balancing valve (STAD) or terminal valve (TBV) This is the general case examined in figure 5.

In figure 6b, each terminal C is provided with a regulating valve (Trim valve or STK). Since they donot allow flow measurement in the terminal units, the presettings of the regulating valves have to becalculated.

In figure 6c, the terminal units are controlled by on-off acting valves with presetting (TBV-C).

Fig 6: One STAP stabilises the differential pressure applied across a set of terminal units.

2.3 One STAP on each control valveDepending on the design of the plant, differential pressure available on some circuits can varydramatically with the load. In this case, to obtain and maintain the correct control valve characteristicsdifferential pressure across the control valves can be stabilised with a ∆p controller as represented on

figure 7.

Fig 7. A p controller stabilises the differential pressure across the control valve.

Notes1- Flow is measured with the measuring valve STAD/M (STAD or STAM), which is an essentia

diagnostic tool.2- When no measuring unit is required (not recommended), the measuring valve can just be replaced by a



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The given values are: the design flow q and the Kvs of the control valve, which is normally known to anaccuracy of ±15%. The theoretical ∆p to be stabilised by the STAP is shown by the following formula:

∆p = (0. 01×q

Kvs

)2

(kPa - l / h)

∆p = (36×q

Kvs)2

(kPa - l / s )

The control valve V is never oversized as the design flow is always obtained for the valve fully open. Thecontrol valve authority is and remains above 0.7.

As the secondary DpL is practically constant, all additional primary differential pressure is taken in theSTAP. The control of differential pressure is quite easy in comparison with temperature control if asufficient proportional band has been adopted to avoid hunting.

Balancing procedure fig 7

1- Open the control valve V fully.2- Preset the STAD/M to obtain at least 3 kPa for design flow.3- Adjust the set point of the differential pressure controller STAP to obtain design flow.

As the flows are correct at each terminal, no other balancing procedure is required.If all control valves are combined with STAP, then balancing valves in branches and risers are nonecessary unless for diagnostic purposes.

Sizing of the control valve.

Sizing the control valve "V" is not critical in this case. It is however recommended to adopt a pressuredrop at least equal to 20 kPa. As we no longer have to follow the rule∆pV > 0.25 pump head, a smalle

∆pV than usual may be adopted, reducing the necessary pump head.

Example with a control valve in injection

Some secondary distributions work with constant flow and variable supply water temperature. A constantwater flow is also required for preheating coils to ensure better freezing protection. For better temperaturecontrol, a constant flow in a unit maintains turbulent conditions and consequently a constant exchangecoefficient. In these cases, a variable supply water temperature is normally obtained with a three-waymixing valve.When the distribution is active (with a primary pump), a three-way mixing valve is not permitted as the

flow can reverse in its bypass, due to the primary differential pressure. When the flow reverses in thebypass of the three-way valve, the mixing function is destroyed. In this case the best solution is to install atwo-way control valve mounted in injection as shown in figure 8a.



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If the primary ∆H varies radically, the authority of the two-way control valve will decrease dramatically,

compromising the stability of the control loop. In this case, the best solution is to stabilise differentiapressure ∆pCD across the control valve with a ∆p controller (Fig 8b). If a minimum flow must be

generated to protect the primary pump, it can be created by a balancing valve situated between C and D(Fig 8c). The adjustment of this minimum flow is quite easy as the ∆pCD is stabilised. Just open this

balancing valve to obtain the minimum required water flow. This minimum flow can also be obtainedwith a relief valve “BPV”. This solution is better than the use of a manual valve because the minimumflow is generated only when necessary. This reduces the primary flow and therefore the pumping costs.

Some designers prescribe a non-return valve in the pipe AB to avoid any flow going from B to A There are two main reasons:

1.

For a preheating coil submitted to a low air temperature, the non-return valve allows theprimary pump to inject hot water in the coil if the secondary pump fails. This ensures aprotection against freezing.

2.

In district heating distribution, if the two-way control valve is oversized or if the secondaryflow is variable, the water flow in the bypass AB may reverse, reheating the returns. The non-return valve avoids this reverse flow.

When such a non-return valve closes, the pressure in B increases, reducing differential pressure across thecontrol valve, improving its authority.

Conclusion

Particularly to reduce pumping costs and compatibility problems at interfaces, variable flow distributionsare used more extensively than in the past. The main problem associated with this distribution is thevariable differential pressure obtained on the circuits and particularly across the modulating controlvalves. Modulating stable control is therefore more difficult to obtain. Variable speed pumps can help, butin a distribution, just one pressure somewhere can be controlled. One reasonable solution is to stabilisethe differential pressures locally with differential pressure controllers STAP with eventually the help of

variable speed pumps.

With differential pressure controllers STAP: modulating control valves work as required, noise isreduced and balancing procedure is simplified.

Ref: Handbook 4 - Hydronic balancing with differential pressure controllers – Tour&Andersson

some application on stap

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