Switch back to simplicity!

Scene 1: It is the early 2000s. I am working with a world leader in smart transmitters. I am having a meeting with a VP of a large refinery. We are discussing ten thousand smart transmitters for an upcoming project. The VP says, "your competitor launched a new pressure transmitter with a better response time!". I reply “Okay, so what is the response time?” - "Well, it is 5 msec better than your transmitter!".

Note! The competitor, in this case, is pitching their transmitter as better and faster!

Observations from Scene 1 - The transmitter vendor is promoting a 5 msec improvement in the response time as a better feature over its rival. There is a delay in analog input scan by the CPU, which is not accounted for. Besides, the response time published by the transmitter suppliers is T63 (63%) and not 100% of the PV. If the transmitter response time is 95 msec added by CPU scan time delay of around 600 msec, totaling to >700 msec. The snap-acting mechanical switches response is 2-3 msec from sensing threshold to switching back to DI. Thus, the switches act 200-300 times faster than a smart transmitter. As my colleague and a very good friend Jan Fatek puts it, if the pump is rotating at 18,000 rpm, i.e., 300 rotations/second, and lube oil starts dripping, and if the smart transmitter takes about 1 second to trip the pump, by this time the pump may have rotated 300 times. The lube oil may heat up and start losing its filming property. To me, there is no comparison of switch v/s transmitter when it comes to response time. Period!

Scene 2: I am in a meeting with a leading EPC company in India. They have bagged two prestigious projects of a fortune 151 ranked state-owned refinery. The Project Manager hands me over the MR (Material Requisition), a bunch of 300 pages with specifications and guidelines for transmitters. I must read line by line, then sign and stamp each page. I see a clause in the MR, "the use of switches is strictly prohibited, The supplier will require to take a prior approval from PMC (Project Management Consultant) to use pressure switches". The PMC and the state-owned refinery had signed MOU in 2001 or 2002 for not using switches in the new projects. The switches weren't part of P&ID from the early 2000s.

Observations from scene 2: The transmitters started replacing the switches in the early 2000s in India.

Scene 3: This is 2009, there is a pre-bid meeting with an offshore platform company. They wanted 700 pressure switches. The meeting room has 7-8 suppliers. I am there with our channel partner. One supplier raises the point that the hermetically sealed micro switch should not be mandatory, it should be optional. The meeting is done, the letters are circulated, and the specifications are changed to non-hermetic SPDT. For an offshore application.

Observations: The specifications were altered to accommodate more suppliers. That offshore platform company is now, preferring transmitters over switches. The reasons are in the roots of engineering practices in scene 3.

Scene 4: It is a prestigious automation exhibition in Mumbai. I walk to a booth set up by an Indian pressure & temperature switch manufacturer. I approach their Booth in charge, the nameplate reads he is their National Sales Manager. Curiously, I ask him a question "What do you think? Will transmitters replace the switches in the future?" He smiles and replies confidently "No!". - “why the switches would stay?”, I asked him another question "what makes you think so?!". He is quick to reply "...because the smart transmitters cost INR 15,000 (USD 200) and switches cost just INR 4,000 (USD 55)".

Observations: There is a need to focus on value selling by all the PS manufacturers.

Scene 5: A large supercritical boiler company in North India, have a project for a state-owned power plant. The pressure switch specs call for, 2xSPDT microswitches with the adjustable dead band. The question - "Do you have two independent set points?" The answer is "No!". The second question is "Do you want a pressure switch for two independent functions at the same set point?" The answer is again negative. I ask the third question "Is there any specific reason, for specifying the 2nd SPDT?" Their reply - "Because the client wants 2nd SPDT as spare in the switch enclosure. The logic is, if the 1st SPDT malfunctions, they will move the wiring to the 2nd SPDT!". This, one SPDT as a spare idea belongs to the era of non-PLC or non-DCS days. The customers had local (field) control with high ampere contacts of about 5A, 10A, or 15A in 60s and 70s. The engineers wanted one microswitch as a spare if the high ampere contacts caused wearing off the microswitch over a period of time. With the introduction of modern PLC and DCS, almost 98-99% of the switches are hooked to digital input cards. There are around 1-2% applications in the remote locations without PLC or DCS with few milliamps of contact rating. Remember, both the microswitches are in the same enclosure and facing the same music of harsh process and ambient conditions, wear and tear. There is a chance when you move wiring from the first microswitch to the second, the second microswitch may not be working.

Observations: with the kind of over-specified pressure switch as above, there will lot of mechanisms involved and the pressure switch would start dancing in a little or high vibration. There are possibilities of frequent setpoint shift/drift resulting in false tripping. The requirement of periodic calibration frequency may go up.

Scene 6 :

A gas cracker plant in a petrochemical complex faces an issue with the pressure switch with a range of 1-9 bar. He has no spare switch in inventory. So, they call their friends in VCM/PVC plant. They have a spare pressure switch unit with the range of 4-45 bar as a spare. "Send it over to me", the Instrumentation engineer calls for an urgent requirement.

Observations: The pressure switches are not modern-day smart transmitters like the turndown of 200:1. Please do not replace a low range PS with a very high range PS. The dead band of the lower range pressure switch is narrower than the higher range pressure switch. Using a high range pressure switch may have a higher dead band, it may require manual intervention to reset the unit.

General note on the selection of level switches:

The usual error in the selection of level switches for liquids is the process data given by the end-user. Make sure you inform the correct specific gravity to your supplier. if the SG is 0.6 and you ask your supplier for a level switch for 0.8, Besides, when to use a displace or a float should be best decided by the supplier, they are the expert on the subject. Please don't specify a float for SG of 0.6 and insertion lenth of 2 mtrs, and ask the supplier to "sign no deviation". The level switch float will nosedive into the liquid. The selection criteria of a level switch is a detailed subject and my aim is to write about it someday later.

Recommendations:

Just Simplify your switch specifications and everything will be fine !

1.      Keep the pressure switch specifications simple. Do not complicate it by over-specifying it.

2.     Replacing switches with smart transmitters is not an overnight job. It requires Management of Change, replacement of the hardware, defining new logic, and sometimes even the hot work permit.

3.     Go for easy solutions like One series 2-wire smart pressure & temperature switches manufactured by United Electric Controls. It doesn’t need hardware replacement; the Management of Change is not required because it is switch to switch replacement. Moreover, with solid-state design and onboard diagnostics, your purpose of a reliable instrument is served. The switch works only on 2-wire, so you can save a huge budget in these pandemic times.

4.     You may insist on a SIL 2 certified and SIL 3 capable pressure & temperature switch for critical service / SIS.

5.     Leave the switch recommendations to the vendors because they are the experts in the business. Let the vendor recommend the sensor, basis on your process information, for e.g. to use a bellow/diaphragm/piston sensor.

6.     Train your technicians and engineers not to treat an electro-mechanical pressure switch like a transmitter. With large volumes of smart transmitters used these days, it is normal to get a grip on pressure transmitter calibration curves, mounting procedures, and calibration frequencies. The handling of a pressure switch is different though simple.