New Pressure Gauge Can Be Read From a Kilometer Away

OK, so the gauge size in the photo is exaggerated just like the exaggerated postcards immigrants to Minnesota used to send back to Sweden to show how fantastic America is. In reality that new pressure gauge is electronic and about the same size as a regular mechanical dial gauge but it can actually be seen from a kilometer away or more because apart from displaying the pressure locally it also transmits the PV wirelessly. By the way, the particular gauge in the photo is made in Minnesota but I will try my best to not pitch a product but instead advance the virtues of automating data collection. Plants are full of mechanical instrumentation, some of which are checked in shift, daily, weekly, or monthly field operator rounds which just isn’t productive. So where do I use a wireless pressure gauge in place of a bourdon tube gauge? Here are my personal thoughts:

Automatic Data Collection

A wireless pressure gauge connects to the same plant-wide WirelessHART network which many sites already have, sharing the network with other wireless devices transmitting the reading wirelessly 1 km or more in a multi-hop mesh to the DCS or direct to the historian where it is trended. This eliminates the need for manual operator rounds to read the pressure. That is, instead of the operator coming to the data, the data comes to the operator. Indication becomes monitoring with trend and alarm on abnormal condition. For any site which does not yet have a WirelessHART network, deploying a wireless gateway is relatively easy.

Indeed, since a wireless sensor network can connect through the HART-IP gateway up across the Internet you can if you like make the reading visible from anywhere in the world provided the proper security is in place: Industrial Internet of Things (IIoT).

One of the major differences between a wireless pressure transmitter and a wireless pressure gauge is that a transmitter has an optional numeric LCD while a gauge has a more visual dial gauge with an indicator needle that can be seen at a glance from a few meters away. This is ideal for applications where a local readout in the field is required. One such application may be at the Local Control Panel (LCP) for starting and stopping pumps or fans in the field. The operators want to see pressure changing when the pump is started and stopped. The gauge may not be located right next to the LCP so the large dial and indicator needle provides an at-a-glance indication.

Using wireless pressure gauges results in more accurate data recording for several reasons. First, a wireless pressure gauge has accuracy of 0.5% of span (grade 2) as opposed to 2% for mechanical gauges. Secondly, a wirelessly transmitted PV avoids the parallax error associated with reading pointer needles on dials. A third source of error associated with manual data collection includes bad hand writing on clipboard sheets which then is read incorrectly, or field operator typing in the reading incorrectly in handheld data collection terminal. The the digitally transmitted reading of a wireless pressure gauge don’t have these issues.

A wireless pressure gauge transmits the PV once every minute which is several orders of magnitude faster than the manual data collection done today. This gives process and maintenance engineers the ability to spot changes early and capture intermittent conditions never before seen.

The fewer personnel there are walking about in the process area, up and down stairs and monkey ladders, climbing under and over obstacles etc., the less the chance of injury such as slips, trips, falls, burns, concussions, and exposure to chemicals. For this reason automatic data collection is a good practice. Also, skipping data collection rounds instead copying the previous numbers have been known to occur during inclement weather. Important early indication impending or intermittent problems can thus be missed.

Reliability

Apart from the automatic data collection there are some additional advantages of going electronic. Mechanical gauges are generally considered “consumables”. They have many failure modes and have relatively short life compared to electronic transmitters. Worst of all, several of the mechanical gauge failure modes are covert, meaning the operator may not realize the gauge has failed so the plant may be operating on a false reading. On the other hand, a wireless pressure gauge is essentially a wireless transmitter with a speedometer on it. Thus a wireless pressure gauge is just as reliable as an electronic transmitter. A wireless pressure gauge has a higher price than a mechanical gauge, just like a computer with software has higher price than a typewriter. However, an electronic gauge is more reliable and therefore a longer expected service life. A wireless pressure gauge may outlast several mechanical gauge replacements. When the price of a few mechanical gauges plus labor cost associated with the replacement is taken into account, a wireless pressure gauge may have a lower lifecycle cost; price is not cost. Wireless pressure gauges is a one-time capital expenditure (CapEx) for the project required to reduce the long-term operational expenditure (OpEx) once the plant is up and running. On a new project use wireless pressure gauges instead of mechanical gauges, but the EPC will not give you wireless instead of mechanical for your pressure gauges on their own accord since they want to minimize cost. This is a plant owner operational philosophy decision which should be taken early on in the project, at pre-FEED in order to get included in FEED, detail design, and so on. 

As I walk down plants I visit I see broken gauges all the time: clouded-over glass, rusty indicator needles bent, indicator needles fallen off, and broken glass. Then there are also covert failures I can't see such as deformed Bourdon tube or worn movement resulting in erroneous reading.

An interesting fact is that Bourdon gauges have many moving parts such as the Bourdon tube, pivot, link, lever, gear, indicator needle (pointer), and spring. The tube and the movement are the main weakness of mechanical gauges. The Bourdon tube flexes and coils as pressure changes resulting in metal fatigue, especially pulsating line pressure. Excess pressure causes the tube to deform giving inaccurate reading. The tube may eventually fail from fatigue or overpressure. Mechanical gauges must be designed to blow-out towards the back in case of failure to reduce risk of personnel injury. The movement also wears due to fluctuating pressure, vibration, and shock. The entire gauge may be filled with oil or glycerin to reduce the effect of vibration, but loss of fill fluid is another failure mode.

A wireless pressure gauge uses an electronic piezo resistive pressure sensor which has much higher overpressure limit than mechanical gauges, up to 150 times of scale range. That is, accuracy of the sensor is does not degrade even if the process pressure is far over range. Similarly the burst pressure limit is much higher for the electronic sensor, over 750 bar before the containment is breeched. No filling is required.

A wireless pressure gauge includes diagnostics to detect sensor failure. Device status and measurement status is reported wirelessly to the control system, historian, or other software. That is, failures are overt enabling quick corrective action. Failure is also indicated locally by the pointer.

Battery powered electronic pressure gauges have existed for a long time but without the ability to transmit the PV to the system. This also means they are not transmitting their status to the system so nobody is notified if they fail. One day when you need to see the pressure the most there may not be any reading any longer because nobody was notified the battery needs replacing. Wireless pressure gauges don’t have this issue because Intelligent Device Management (IDM) software notify instrument technicians well in advance if the battery is running low which is once every 10 years.

There are several other types of mechanical instruments in a plant such as temperature gauges, variable area flow meters, positive displacement flow meter, and sight level glasses etc. These mechanical devices have similar issues as mechanical pressure gauges. It may be a good idea to replace these with the corresponding wireless transmitters.

Plant Modernization

For existing plants you can either replace gauges one by one as and when they fail, but best practice is probably to replace them in a campaign. Start by replacing the gauges which are read in the daily field operator rounds. Simply look at the field operator check sheets to see which they are. Mechanical gauges located in unsafe areas where personnel movement should be minimized should also be replaced with wireless pressure gauges. For instance, if there is any area which has potential for HS gas, then those gauges are prime candidates for going wireless. Mechanical gauges in applications seeing high failure rates can be replaced with more reliable wireless pressure gauges. Conduct a plant modernization audit to identify mechanical devices to be replaced. See further ideas in this article:

https://www.ceasiamag.com/2015/04/instrumental-to-success/11137/

So, a wireless pressure transmitter has an optional numeric LCD while a wireless pressure gauge has a more visual dial gauge with an indicator needle that can be seen at a glance from a few meters away. My question to you is; in which applications do you need a large dial gage which is displaying with a indicator needle on a scale as opposed to using LCD? If you have some application ideas, please share below. Well, that’s my personal opinion. If you are interested in how the digital ecosystem is transforming process automation click “Follow” by my photo. Click “Like” if you found this useful to you and share it with others if you think it would be useful to them.