Bridge or No bridge for primary/secondary pumping (re-post) 1

[cry22]

Hello all.
I posted this thread in the "pump emgineering forum" and was advised to post here instead. I thought folks visiting pumping forums wre better qualified, but seems like I am speaking chinese to some (biginch). Hoping to get some debate here instead.

I have seen several arrangements in primary/secondary pumping systems.
Some utilize a suction from the primary and return to the primary with no devices of any kind.
Others use a bridge with a control valve.
Others use a 3-way valve.

I have used successfully primary/secondary pumping without using a bridge at all. Am I missing something? should I use a bridge? a 2-way control valve? whether in the decoupler or at the suction.

What is your pratice out there? any problems with any system you used? How reliable is the bridge and what does it add to the system?

Is it worth putting a bridge altogether? and why?

Thanks

 

[imok2]

BELL AND GOSSET
Primary-secondary pumping is simple in theory as well as operation. It is based on a simple fact: when two circuits are interconnected, flow in one will not cause flow in the other if the pressure drop in the piping common to both is eliminated.
RULES OF THUMB
#1 THE COMMON PIPE
The key to all primary-secondary applications is the use of a common pipe which interconnects the primary and secondary circuits. The length of this pipe should be kept very short in order to keep the pressure drop very low, and the supply and return tees to the secondary circuit should be a maximum of four pipe diameters apart. By keeping the pressure drop very low, water that is flowing in the primary loop will not flow into the secondary circuit until its circulator turns on.
#2 THE SECONDARY CIRCULATOR
A separate circulator is installed in the secondary circuit to establish flow. This circulator is sized to move the flow rate and to overcome the pressure drop of its circuit only. The circulator should be located so it is pumping away from the "common piping" and discharging into the secondary circuit. This causes an increase in pressure in the secondary circuit rather than a reduction in pressure which would occur if the pump were located on the return pumping towards the common pipe.
#3 THE LAW OF THE TEE
This rule determines the flow rate and direction of flow that occurs in common piping. It is based on the relationship of the primary and secondary flow rates, and there are three possibilities to evaluate:
1.Primary flow more than secondary
2.Primary flow equal to secondary
3.Primary flow less than secondary
This rule of thumb is best described by a simple statement: flow into a tee must equal flow away from the tee.
#4 FLO-CONTROL VALVES
Flo-Control valves are recommended to prevent any flow into the secondary circuit induced by either the slightest pressure drop that may exist on the common pipe or by gravity heads. Because gravity flow can occur within a single pipe, two Flo-Control valves are best, one on the supply and one on the return. However, if the secondary circuit's return is underslung, only one valve is needed.

 

[marcoh]

in addition to Imok2's comments bridge's in conjunction with control valves are also used for variable temperature control, ie if the primary circuit is constant temperature and the secondary circuit has a variable temperature requirements.

Bridges may also be used if you want to be able to control start-up flow issues. Had a look at a large primary/secondary/tertiary campus system recently where the bridge or bypass was installed to solve tertiary pump startup issues with flow balancing while the secondary pumps caught with the the tertiary pump demands.

 

[cry22]

Thanks guys.
One question: In the decoupler, do you install a flow meter? a bi-directional flow meter.
I have seen such an application where a bi-directional flow meter in the decoupler is used in conjunction with the temperature sensors and delta T to stage the chillers, i.e. when flow is sensed in the opposite direction in the decoupler for 10 minutes (adj), it is time to bring another chiller (or a larger chiller) on line.
This brings the question of asymetric chiller plants worth discussing.

Thanks again.

 

[ChasBean1]

That's a pretty interesting way to stage chillers but I don't know if you really want to do that. There could be times when secondary flow is a little higher than primary flow with temps satisfied and chillers not over capacity.

The simplest way to stage up is CHW supply temp 2° greater than set point for X (typ. ~30) minutes.

Not a good idea to try to stage back down on temp (or dT) however... better off using tonnage or %load from the chiller panel.

 

[RossABQ]

I am familiar with very large (10+k tons) systems that use the control logic for sequencing described above (flow meter in bridge). It is used in these cases where a 2-deg bump in primary temp is not acceptable for the processes served. System stability requirements override all other factors in this case. It seems to work very well with proper PLC programming. The system loads include a large constant-volume OSA conditioning component, so there is a daily chiller sequencing that occurs as OSA temps go from say 40 to 75 during the day, and the sequencing is very orderly.

 

[cry22]

Chasbean
Don't agree with that method so much, we've seen so many plants with low delta T using the set up you describe.
Is is not a good indicator of the plant load.

Delta T optimization has always been problematic, some go as far as putting a circulating pump across the CHW coil to maximize the delta T before they send back to teh chiller plant.

CHW supply/return plus flow in secondary piping with a check within the decoupler of flow direction tends to indicate a better load profile overall.

 

[ChasBean1]

Cry22, pls. don't put a check in a decoupler!

You don't agree with what method so much? Didn't say anything advocating using a delta-T. Just wondering if you might have misinterpreted my last post.