Méthode compensée

The Compensated Method is a further development of the Proportional Method, with three main advantages:​ ​

Staged commissioning: You can balance the plant in stages as construction goes on, without having to rebalance the entire building when it is completed.

Quicker commissioning: It reduces time consumption significantly since it is not necessary to measure the flows in all balancing valves and calculate all flow ratios. It also requires just one flow adjustment at each balancing valve.

Minimised pumping costs: When balancing is finished, you can read off the pump oversizing directly on the main balancing valve. The pump head may be reduce​d correspondingly. Frequently, large energy savings can be made, particularly in cooling plants.

1. A development of the Proportional Method
The Compensated Method is based on the Proportional Method, but is further developed in
one essential aspect: Using the Compensated Method, the flow ratios are automatically kept
equal to 1 throughout the balancing process of a module.
a) Staged commissioning
  • The plant may be divided in modules. This means that the plant can be commissioned in stages, as construction goes on, and no rebalancing of the entire building is required after completion.
b) Quicker commissioning
  • ​No first scan to measure the flows in all branches and risers. No calculation of flow ratios to determine the starting point of balancing.
  • Balancing can start at any riser (although you should close the risers you are not balancing).
  • ​No worrying about causing a too high flow for the main pump. No worrying about the differential pressure being too small to produce measurable flows.
  • Only one flow adjustment at each balancing valve is required.
c) Pumping costs may be minimised
  • ​The Compensated Method automatically minimises pressure losses in the balancing valves. The main balancing valve reveals any oversizing of the main pump. The pump may often be exchanged for a smaller one.
  • ​The set point of a variable speed pump can be optimised.

2. Reference Valve and Partner Valve
When the flow is adjusted by a balancing valve, pressure losses change in the valve and pipe line, thereby changing the differential pressure across other balancing valves. Flow adjustment in one balancing valve thus changes the flow in valves that have already been adjusted. This often makes it necessary to adjust the same balancing valve several times over.

The Compensated Method eliminates this difficulty. The flow in each balancing valve is only adjusted once. The method assumes that it is possible to measure the flow disturbance occurring when a balancing valve is adjusted, and that the disturbance can be compensated in some way.

The disturbance is detected on the balancing valve furthest away from the pump, in this module. This balancing valve is called the Reference Valve.​
A balancing valve acting on the total branch flow, called the Partner Valve, compensates for the disturbance. With this valve, the differential pressure across the Reference Valve can be reset to its initial value each time a disturbance occurs.

The method begins by adjusting the flow to design value in the Reference Valve, according to a particular procedure presented below. The result is a certain differential pressure ΔpR (Fig 1), which is to be monitored continuously. The Reference Valve is then locked to this setting.​



Since the flow is now correct, the pressure losses are also correct in terminal 5, its balancing valve and accessories. The differential pressure ΔpEH is therefore correct and we may proceed to adjust the flow in terminal 4. When the flow in terminal 4 is being adjusted, ΔpR changes slightly in the Reference Valve, whose setting is locked. This is an indication of the disturbance from the flow adjustment in terminal 4. ΔpR must be readjusted to its initial value with the Partner Valve. In other words, design flow must be readjusted in the Reference Valve by compensating on the Partner Valve.

Since the flows in terminal 4 and 5 are now at their design values, the differential pressure ΔpDI across terminal 3 is also equal to the design value. The flow in this terminal may therefore be adjusted. Adjustment of the flow in terminal 3 creates a disturbance, which is detected at the Reference Valve and compensated on the Partner Valve. The readjustment of design flow in terminal 5 automatically brings the differential pressure ΔpEH and the flow in terminal 4 to design value.

This procedure works well regardless of the number of terminals on a branch. Adjustments must be carried out by working towards the pump, beginning at the Reference Valve. The same procedure is then applied for balancing of risers. The last branch on the riser furthest away from the pump is used as the reference, and the riser’s balancing valve becomes Partner Valve.


3. Setting the Reference Valve
Select ΔpR as small as possible but big enough to meet the following two conditions:

  • Minimum of 3 kPa to obtain sufficient measurement accuracy.
The CBI balancing instrument indicates flow for differential pressures down to 0,5 kPa. However, to decrease the relative influence of the pressure pulsation in the plant on the flow measurement, we recommend ΔpR > 3 kPa.

The Kv value may be calculated for a pressure loss of at least 3 kPa using the formula:

Kv = 5.8 q (m3/h) or Kv = 21 q (l/s)

Another and simpler way, is to let the CBI calculate the correct setting of the Reference Valve.​

  • The pressure drop in the valve fully open and at design flow.
If the pressure loss is greater than 3 kPa for design flow and the balancing valve fully open, it is obviously not possible to set the Reference Valve to create 3 kPa. This represents the second condition on the ΔpR: at least as high as the pressure loss across the fully open balancing valve at design flow. In this case, the balancing valve on the reference is just fully open.

When a suitable ΔpR is selected, preset the Reference Valve to create ΔpR for design flow. Use the CBI or a nomogram to find the correct handwheel setting. Then lock the handwheel.

To obtain the selected ΔpR, and thus design flow, adjust the Partner Valve. This is always possible since the other risers are closed and the pressure loss in the main pipe line is small. The available differential pressure is thus higher than normal. The surplus will be taken in the Partner Valve.

If the pressure losses differ substantially between the terminals, please refer to section 8.

4. Equipment needed
Two CBI balancing instruments are needed to measure differential pressures and flows in the balancing valves.


5. Balancing terminals on a branch
Select any riser, for instance the one closest from the pump. This ensures a sufficient differential pressure for the selected riser. Select any branch in the riser you have selected. Normally, you do not have to shut any of the other branches of this riser. However, if some branches are provided with a bypass line, which can create short circuits, the flow in these branches has to be limited or these branches isolated.

  1. Determine which is the handwheel position of the Reference Valve that will give design flow at the selected ΔpR (normally 3 kPa). Use the CBI or a nomogram to find the correct handwheel position.

  2. Adjust the Reference Valve to this position and lock the valve (turn the inner spindle down to stop).

  3. Connect one CBI to the Reference Valve.

  4. Balancer (1) adjusts the Partner Valve to obtain the selected ΔpR in the Reference Valve. Information about current value of ΔpR is transmitted to Balancer (1) from Balancer (3) by means of a walkie-talkie for instance. This operation gives design flow in terminal 5. If the selected ΔpR cannot be reached, the cause may be that non balanced terminals on thbranch receive a too high flow. Shut as many of them as required to obtain the selected ΔpR.​




  5. Balancer (2) now adjusts the flow to design in terminal 4 by using the CBI computer function. It calculates which handwheel position that will give design flow. During the​  whole procedure, balancer 1 continuously readjusts the partner valve to maintain ΔpR to its initial value.

  6. Balancer (2) adjusts the flows in each terminal by working successively towards terminal 1, according to step 5 above. All terminals on the branch are now balanced relative to each other, independently of the current differential pressure applied on the module. 
Note: Let us suppose working with two balancers (1 and 2) and two CBI (CBIa and CBIb). When adjusting terminal 3 for instance, with CBIa, the balancer can check the evolution of the flow in terminal 4 (CBIb) instead of going to the reference (terminal 5). He communicates with balancer 1 to readjust the flow at terminal 4, takes back the CBIb put on this terminal and eventually he readjusts the flow at terminal 3. He leaves the CBIa put on terminal 3 and goes with CBIb to terminal 2, following the same procedure and checking the flow evolution at terminal 3.​


6. Balancing branches on a riser​



  1. Find out which handwheel position for the Reference Valve STAD-1.9.0 that will give design flow for the selected ΔpR, normally 3 kPa. Use the CBI or a nomogram to find the correct position.

  2. Adjust the Reference Valve to this handwheel position and lock the valve (turn the inner spindle down to stop).

  3. Connect one CBI to the Reference Valve.

  4. Balancer (1) adjusts the Partner Valve to create the selected ΔpR in the Reference Valve. This then gives the design flow in the reference branch. If the selected ΔpR cannot be obtained, the cause may be that some branches on the riser receive a too high flow. Then close as many branches as required to obtain the selected ΔpR.

  5. Balancer (2) now adjusts to design flow in branch 1.8.0 by using the CBI computer function. It calculates which handwheel position that will give design flow. During the whole procedure, balancer 1 continuously readjusts the partner valve to maintain the flow in the reference to its initial value.

  6. Balancer (2) adjusts the flows in each branch by working successively towards branch 1.1.0 according to the procedure in step 5 above. All branches on the riser are now balanced relative to each other independently of the current differential pressure available on the riser.

5.7 Balancing risers on a main pipe line​

 


The balancing procedure is exactly the same as for balancing of branches on a riser. The Reference Valve is now STAD-7.0 and the Partner Valve is STAD-0.
When balancing of risers 7.0, 6.0, 5.0 etc., is completed, the entire plant is balanced for design flows and the remaining pressure loss in STAD-0 reveals the pump oversizing. If the excess pressure is large, it may be profitable to change the pump for a smaller one.

When using a variable speed pump, the STAD-0 is not necessary. The maximum speed is adjusted to obtain the correct design flow in the Partner Valve of one riser. All the other flows will be automatically at design value.


8 Setting the Reference Valve when pressure losses differ substantially between the terminals
If the terminals create pressure losses that differ substantially, a ΔpR of 3 kPa in the Reference Valve may not be sufficient to give the necessary differential pressure for the other terminals. This problem is solved in the Proportional Method by using the same flow ratio for the Reference Valve as the flow ratio measured in the index circuit. But the Proportional Method often overestimates the ΔpR and balancing is not optimised (unnecessarily high pressure loss in the balancing valves). A way to achieve a suitable value for ΔpR is presented below. 
The branch in figure 6 has terminals with different pressure losses.​



Select ΔpR based on recommendation in section 3, normally 3 kPa. We call this preliminary value ΔpRo. Proceed with balancing according to the Compensated Method.

When you reach the index circuit, you will note that it is impossible to obtain design flow since the differential pressure is only 29 kPa, while it would take more than 40 kPa to obtain design flow. Perform the following steps:

​           7. Shut the balancing valve (V2) of the index circuit and readjust the correct flow in the reference with the Partner Valve. Measure the differential pressure across V2. Call this value Δpo.

           8. Preset V2 so that its pressure drop will be approximately 3 kPa for design flow.

           9. Open the partner valve to obtain the design flow in the index circuit.

         10. Measure the flow in the reference circuit. Calculate the flow ratio λ = flow measured/design  ​                      flow.       

         11. The new value of ΔpR to be set on the Reference Valve is given by the formula: 

              

        12. Preset the Reference Valve to obtain this pressure loss for design flow, and rebalance the entire  ​               branch.


Compared with fig 6, the result of this procedure is given in fig 7.​



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Méthode compensée