Pressure transmitters – ten top tips for top performance
Key points
Pressure transmitters are widely employed in wastewater treatment, pulp and paper production, and many other applications. But how do you get top rate performance from such a vital piece of equipment?
Here we present ten top tips for maximum measurement accuracy, with the minimum of maintenance headaches.
Top tip #1 – Maintain a tight fit
The diaphragm in a pressure transmitter is a very thin metal foil. Keeping it clear of deposits will ensure greater measurement accuracy. Unfortunately, some process applications, such as wastewater, are particularly prone to clogging. This can be overcome using a flush or front bonded diaphragm to create a tight fit between the pressure device and the pipeline.
Top tip #2 – High or low? The right mounting point is crucial
The right mounting position is key to effective pressure transmitter operation, as gas bubbles accumulate at the high points. This means that for gas applications, the transmitter should be mounted at the high point of the flow, while a low point is better for clean liquid or steam applications.
Top tip #3 – Get the right level
When using pressure transmitters for level, consider where the pressure transmitter is located. It must be mounted at the low level of fluid or below to provide maximum working range. Fluid in the tank below the bottom mounting point can also be factored into the range of the device during setup.
Top tip #4 – Keep manifold valves in one place
Manifold valves allow the pressure transmitter to be isolated from the process and also isolate and equalise the pressure in a transmitter, for example to check the zero value.
It’s good to mount the manifold to a wall or pipe and not the transmitter. This makes removing the pressure transmitter easier for calibration or cleaning.
Top tip #5 – Choose friendly fill fluids
The choice of fill fluid in a pressure device can be important. It needs resist the effects of temperature to prevent expansion or contraction. In particular, food applications require an FDA approved fill fluid to protect against contamination from fluid leaks.
Some process fluids, like oxygen or chlorine, are highly volatile – for these, an inert fill fluid is vita
Top tip #6 – Keep it on the shady side
Pressure transmitters should be located away from direct heat to avoid temperature effects.
Depending on the pressure differential range of the pressure transmitter, heat can greatly affect the device’s ability to measure pressure difference. Potential solutions include using a sunshade to deflect sunlight or locating the device somewhere with low ambient heat.
Top tip #7 – Keep impulse lines in check
Differential pressure measurements are achieved by using either remote seals or impulse lines – these should be parallel and the same length to minimise errors.
Impulse lines should also be short to discourage blockages. If frequent blockages are likely to occur, consider using a remote seal design instead.
Top tip #8 – Trust venturis
Differential pressure is used for measuring flow using a venturi tube, a shaped piece of pipe that narrows in the middle and widens at either end. To improve the turndown, the ratio between the maximum and minimum range of the device, you can ‘stack' up to three pressure transmitters to increase the working range. Trusted and reliable, a venturi is typically much more cost-effective than a high specification ultrasonic device.
Top tip #9 – Be aware of wear
Lightweight pressure diaphragms can be a problem in abrasive applications. One solution is to use ceramic rather than metal diaphragms – however, these can be brittle and prone to failure. To overcome this, ABB has developed Diaflex®, a flexible diamond coating that offers maximum resistance to abrasion.
Another challenging process medium for pressure transmitters is hydrogen gas, which can eventually render the pressure transmitter useless. ABB has developed H-Shield®, a protective uniform coating that covers the surface of the diaphragm and provides the highest resistance to permeation of hydrogen.
Top tip #10 – Keep turndown in perspective
Turndown is often misunderstood. Temperature, for example, could have a greater effect on transmitter performance depending on the sensor range and the turndown – if the temperature affects pressure by 1 mbar, then on a 100mbar sensor this would be 1 percent. However, when at the bottom end of the turndown of this sensor, say 10 mbar, the temperature effect of 1 mbar would have a much more significant effect on accuracy than the turndown.
For further information, please visit here
ABB Measurement & Analytics
- 08706006122
- enquiries.mp.uk@gb.abb.com
- http://www.abb.com
- Howard Road Eaton Socon Saint Neots PE19 8EU GB
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