Liquid differential pressure responds

[ElGr]

We are changing chilled water system control strategy from head pressure to DIFFERENTIAL Pressure control between flow and return on the index leg at the most hydraulically remote locations to the VSD Secondary chilled water pumps.
Differential pressure transmitter which was preliminary selected is Siemens QBE3000-D6 with range 0-6bar (0-10V output to suite our QBMS panel)
However, there are concerns about differential pressure transmitter responses in case of negative pressure in supply line which may happen at the start of cycle. (if the cooling load applied to AHUs field cooling coils valves (2-way) will open and (VSD Pumps are off) the head pressure in supply line can be negative)
I have contacted Siemens. The response Siemens gave me “In the situation of negative pressure nothing happens. Mechanically the sensor will be not damaged of course. The electronics are not sealed at the zero point. There is no usable signal available. In the case of 4 ... 20 mA the output can be min 3.7 mA ... 3.8 mA depending on component tolerance. The voltage output will remain at 0 volts”
However, The question still remaining: how differential pressure transmitter will determine differential pressure between flow and return if voltage output will remaining 0 at variations of negative pressure ?
Has anyone had this experience in using Liquid differential pressure responds? Changing closed loop system control to differential pressure control?
Could you please help and confirm in case of negative pressure at “high” port and if the voltage output remain at 0V how differential pressure transmitter will determine differential pressure between flow and return????
Would anyone have by any chance a video of liquid dP responses on your site?
Thanks
Regards

 

[danw2]

DP transmitters work by subtracting the low side pressure from the high side pressure and reporting the difference.

A DP transmitter ranged 0-6 bard is unipolar and will only report pressures when the high side pressure is greater than the low side pressure. If the low side pressure exceeds the high side pressure, Siemens' assertion that it will not damage the unit is likely correct, but the transmitter can't report a bipolar differential measurement that is 'negative' when it is a unipolar instrument.

You could buy two QBE-3000's and pipe/plumb one of them 'backwards', normal high side pressure to the low side sensing port and normal low side pressure to the high side sensing port which would have a useable signal only in the condition for reversed pressures, at start-of-cycle (if that's what really happens, I don't know; I'm not an HVAC guy, so your app is black magic to me).

If that's too out-of-the-box, consider a bidirectional DP transmitter, like Siemens' industrial DP DSIII series transmitter. It will measure and cover a range, for instance, of -5 bard to +5 bard over a 4-20mA range,
where 4.0mA = -5.0 bard
12.0mA = 0 bard
20.0mA = +5 bard

The next range up is ±30 bard, but that's within the 5:1 turndown that smart transmitters are capable of.

The DSIII's are all 2 wire, loop powered transmitters. Any brand 2 wire transmitter needs 3.5mA at 24Vdc to operate its electronics, so the output is always a live zero; there's never a 0 - 10Vdc output (0-10Vdc is a 3 wire device, the third wire for a DC power supply) for a 2 wire transmitter.

For a system to deal with a voltage input and an elevated zero, put a 500 ohm resistor across your system's analog input which converts the DC current to a DC voltage. The system will 'see' 2.0V - 10.0V for the span ±5 bard.

The system span would be -7 to +5 bard (20% of a 10 bar span below the lower range value for the 0-2Vdc input span). You'd never see -7.0 to approx -5.22 bard, those values being outside the range of the source 4-20mA input.
Industrials cost probably 2-3x what the QBE-3000 costs.

For a Siemens DSIII, you'd need to specify a Y01 suffix in the transmitter's model code if you want a transmitter calibrated over the entire bipolar range, both negative and positive pressures (factory default is positive range only). In your case, option suffix Y01 = -5.0 bar to +5.0 bar for the 5 bar model or -6.0 to +6.0 bar for the 30 bar model.

 

[DRWeig]

Perhaps I'm missing something -- but why would you be concerned about differential pressure across a coil when the pumps are off?

Best to you,

Goober Dave

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[ChasBean1]

Dr. Weig, you might be missing something because of the presentation of the question.

ElGr, although you were articulate in your post, you need to be clearer. A head pressure control might modulate condenser water flow or temperature to ensure full refrigerant condensation without subcooling.

Your statement “DIFFERENTIAL Pressure control between flow and return on the index leg at the most hydraulically remote locations to the VSD Secondary chilled water pumps” left me to read your post five times without understanding what the f*&^ you mean.

“there are concerns about differential pressure transmitter responses in case of negative pressure in supply line which may happen at the start of cycle..”
It sounds interesting, and I’d love to help, but no way until you figure out how to ask your question more clearly.

 

[urgross]

ChasBean,

I was going to answer, as the configuration and position of the dP's suggests moving to variable primary pumping. I've done that. Never seen a dP going to negative and not sure how it can occur unless installed improperly (reversed) across the indexed coil. Most 2-ways will require about 6 psig to operate, so I've never even seen that condition in theory.