Question { IOCL, 8583 }

Yokogawa mass flow meter shows negative reading...what r the main causes for this problem?...

Answer

Yokogawa mass flow meter shows negative reading...what r the main causes for this problem?...

Impossible to say, because the technology is not stated, nor are the conditions stated.

Yokogawa is a brand name, not a technology, and the reason for a negative flow indication depends on the technology.

Technologies for mass flow (however inferred) are
- multivariable flow meters based on pressure and temperature compensated differential pressure
- Coriolis flow meters
- Vortex flow meters with temperature and pressure compensation
- Thermal dispersion flow meters
which might or might not be Yokogawa products.

The reasons for a false negative flow indication could be quite different for any of the technologies

Conditions like the expected flow rate vs the observed flow rate, medium, medium temperature and pressure, service history, installation details (vertical or horizontal), flow path, type of pump, process function or contiguous piping and its orientation, and configuration elements like low flow cutoff could contribute to an indicated negative flow rate, or operational method of zeroing.

Because it could be that you actually have reverse flow from a thermal syphon or a pressure syphon.

Carl Ellis
Measure First

Question { Rosemount, 3360 }

How we can re-range the DP transmitter, range is -0.5 to 2.0bara. new range is -0.5 to 5 bar

Answer

Something is wrong with the stated situation. The stated range is in -0.5 to 2.0 bara, where bar is bar absolute.

An absolute pressure references absolute zero (a hard vacuum).

No measurement can be made below zero bar absolute because it violates the laws of physics.

While a DP transmitter can not make a negative absolute measurement, it can make a negative differential measurement where the low side pressure is higher than the high side pressure.

The method of changing the differential range depends on the brand/model of the transmitter.
- zero and span adjusting pots
- zero and span push buttons
- local indicator/push buttons
- HART communicator/modem-software

The first two require an applied reference pressure for zero and span.

The 3rd and 4th options might have a choice of applied pressure or entering values for zero and span.

Some DP transmitters do not have the same accuracy in the negative range as they do in the positive range.

Carl Ellis
Measure First

Question { Reliance, 2889 }

how to calibrate pressure gauge(we can not able to fixed by pointer screw adjustment and pointer re fixing method)

Answer

You need to disassemble the gauge to see if the movement and its linkage is broken or bent. The note below covers general gauge repair. If the zero adjust does not cover the range, a span adjustment might fix it but it is more likely that either the movement linkage is broken or the Bourdon tube has been damaged and is no longer serviceable.

Brief Overview of Pressure Gauge Repair

Pointer off its shaft
The pointer sometimes fall off under vibration conditions. The pointer can usually be replaced on the pinion shaft at least once. Severe vibration wears out all the components in the movement, eventually jamming the gauge. Liquid filling a gauge dampens vibration effects and prolongs the life of gauges.

Pointer sticks during motion
Linkage bearings can become gummed up and require cleaning, just mechanical clocks and watches require cleaning of bearings. Linkage can break (particularly under vibration) or wear to the point of jamming. Dropping a gauge can bend or jam linkage.

Pointer does not move or has very limited movement
Clogged socket or Bourdon tube
The Bourdon tube could be clogged with some substance so gummy that it prevents free movement. It is an economic decision as to whether cleaning the tube and socket is worthwhile, given the time and material costs. Be warned that a brass Bourdon tube will be damaged and weakened to the point of failure by the application of any cleaning agent with ammonia. Care should be taken not to stress bend the Bourdon tube while handling or cleaning.

If the gauge has been dropped, linkage parts might be bent out of alignment which can jam or seize the movement action.

Vibration wears and fatigues the internal linkage and movement. The bearings or gearing might have failed and jammed. Unless a replacement movement is available, we consider the gauge to be not repairable.

Adjustments
If the socket and tube are clean and the movement moves freely with no broken parts, a known pressure (roughly mid-scale) is applied to the gauge with a dead weight tester or is applied from a pressure source and monitored with a certified 'test' gauge; both are (relatively) expensive devices. Adjustments are made to for span and zero.

Zero Adjustment
A known cause of a negative offset in liquid filled gauges is when the case vent is stuck closed. Vent the gauge with the vent located somewhere near the top of the case so that positive or negative pressure does not build up inside the case and cause an offset.

The zero adjustment method depends on the gauge design/construction (also a relative indicator of quality). The methods include
- adjustable pointer
- cam adjustment in movement
- dial rotation
- pointer removal and replacement at a different position on the pinion shaft
- none

An adjustable pointer provides about 10% (full scale) adjustment.

Gain, Fast/Slow adjustments
Gain or Fast/Slow adjustments can be made by various adjustments - slotted links, adjustable length links, U or O (horsetrack) links that are squeezed close (to shorten) or open (to lengthen), to name a few.

Calibration NEVER involves bending or repairing the Bourdon (C shaped) tube or socket. A Bourdon tube that has been over-ranged is stressed to a point that it will not indicate the pressure accurately and it can not be fixed. A Bourdon tube that leaks can not reliably be repaired and needs to be discarded.

2 informative Youtube videos:

Adjustable pointer on Ashcroft process gauge
https://www.youtube.com/watch?v=VbAke-0ijws

How to Reset a Liquid Filled Pressure Gauge
https://www.youtube.com/watch?v=SvpsWjan6Wc

Obtaining parts for all but the highest quality gauges can be problematic.
Most gauge manufacturers no longer offer repair parts for gauges, although some do for the highest level quality gauges.

Carl Ellis
Measure First