We don't use switches

I've heard this ten times a year since 2001, so it should now be equivalent to the number of pressure and temperature switches in a medium-sized plant! I've spent my entire career selling float-type/displacer-type level switches, flow switches, pressure, DP, temperature switches, and smart transmitters. The organizations I worked for, had high standards for training programs.

The route cause of any instrument malfunctioning in the application lies in specifying the wrong "specifications". The error is followed by cementing the error taking a strict bid evaluation criteria (BEC) with a statement in the bid "Any deviation will not be accepted" or "any deviation to the specifications will result in rejecting the offer" or "this is a fast track project and vendors confirming the specifications without any deviation will be given preference".

I have a suggestion that, leave the model selection in the scope of vendor because they are the best to offer the specifications as per the application. My response to an incorrectly specified pressure switch is "we cannot comply with an incorrect specification just simply signing "no deviation"!"

We cannot confirm the wrong spec to stay alive in the bid! For a spec that is a misfit for the process conditions.

It's like creating a problem and then solving it.

How the problem develops, example No. 1, which unnecessarily specifies 2xSPDTs:

Specifying 2 x SPDT micro switches even if there is one set point and no second action is needed. The reason, the user would wire only one SPDT and keep the second SPDT as a spare.

Logic - if the first SPDT runs out of its life, the wiring can be shifted to the next (spare) SPDT micro switch in the same pressure switch. This logic was used during non-PLC/DCS days where the control was local and direct on the final control element with high ampere contact AC ratings.

What kind of problem was created? - with an extra micro switch, there came an additional mechanism in the switch, making the switches prone to vibration and process spikes. Some vendors offered 1xDPDT because they had no option of 2xSPDTs, the end user paid for DPDT instead of just 1xSPDT. The budget went out of their small annual maintenance pockets. With 2 x SPDTs in the same pressure switch, both micro switches face the same music of harsh process environment, vibration, process pulsation, and ambient effects. It is just that one is wired and the other is not. The chances are, that when you move the wiring to the next microswitch, the "spare" microswitch is already dead. With modern PLC / DCS and implementation of IEC 61511/61508, about 99% of the pressure & and temperature switches are hooked with digital input cards rated 10-30 VDC/20 mA.

Consequences of this over-spec'd switch:- False alarms, trips, downtime, and probable production loss.

How was the problem addressed? - with "We use only smart transmitters" !

How the problem was created, example No. 2 : Selecting the wrong micro switches.

There is an offshore company, listed in Fortune 500. For one of their platforms, they have a requirement of around 600 nos. pressure switches. They have published a tender with specifications. To give equal opportunity to the bidders, they call for a pre-bid meeting of the bidders (suppliers). One of the supplier raised his objection that the specs call for "gold plated hermetically sealed suppliers micro switches" which favors certain manufacturers and it has to be changed to non-hermetic. The buyer agrees to tweak the specs in the final version of the tender document. Probably, afraid of the fingers pointing at him favoring certain vendors?! I witnessed this incident in the pre-bid meeting.

Consequences: Eventually, the competent vendors pulled out of this tender because a low-cost vendor paved the way. The non-hermetic micro switches with silver plating would get oxidized in the salty offshore climatic conditions and eventually, the pressure switch would not perform its duty. This Fortune 500 company later learned its lesson and removed that brand from its AVL.

How was the problem addressed? - "We use only smart transmitters" !

How the problem was created, example No. 3 :

Unlike smart transmitters, you have to be careful in the interchangeability of a mechanical pressure switch or DP switch.

The Captive Power plant of a petrochemical complex had a problem in a pressure switch with a range 0.5 - 7 Bar, the instrument technician called the Instrumentation technical of the Chlor Alkali plant to check if he had a spare pressure switch, and the Chlor Alkali plant technician sent him a pressure switch with the range of 2.5-55 Bar.

The technician of CPP installed the switch with a set point of 3 bar falling pressure. The switch has a changeover at a given set point but it doesn't reset when pressure builds up! The technician concludes, he got a bad pressure switch. No! The replacement pressure switch has a higher dead band compared to the process application. The interchanging is not done keeping in mind that the pressure switches are unlike smart pressure transmitters, that you can range down 10:1 and start using it in a new application but more engineers are trained on smart transmitters, they consider the pressure switch has the same capability.

Staying rigid to selecting the type of sensors:

Better leave the model selection to the vendor's scope, they are the experts on their products. The vendors would be best to judge if a pressure switch with bellow would be appropriate or a diaphragm or a piston sensor. It all depends on the requirement of dead band, over-range pressure and proof (design) pressure requirements. Some buyers specify bellows for high pressure and proof pressure requirements, where it is better to use a diaphragm or piston sensor but rejecting an offer from a vendor because they didn't offer "the sensor, bellows in this case, that was specified".

How was the problem addressed? - "We use only smart transmitters" !

Example 4 - Creating a problem in level application :

A top internal level switch with a perforated guide pipe and insertion depth of 2000 mm for a liquid with a specific gravity of 0.65 was specified as a float type. The vendor wanted to suggest a displacer-type switch. The PMC (project management consultant) didn't agree because it was a fast-track project and no deviation from the original specs was allowed. The vendor offered a float size of 150x90 mm size because it required a bigger float for better buoyancy force. Eventually, the float was bigger than the internal gauge of the flange nozzle. The engineers at the site hammered the float to insert it inside the nozzle and the float became out of shape. The switch was replaced with a displacer-type level switch by the Manager of Instrumentation at the end user side. I have witnessed this incident.

How was this problem solved? : With the increased use of guided microwave radar level transmitters across all industries.

Example 5 - Specifying "no interface" with two specific gravities.

A large PMC in India specified level switches for separators, the datasheet mentioned two specific gravities. The upper liquid is hydrocarbon with 0.6 and the lower liquid is water 1.0. But the column of "interface" mentioned "No". the vendor got confused and under the pressure of confirming "no deviation", offered the float type level switch considering the upper SG of 0.6. Now, the function of the level switch was to drain water to safe drainage but it sensed upper SG of 0.6 and it started draining hydrocarbons in the drainage, thus creating a possible hazard. Later, the end user of the switches replaced all the float-type level switches with the displacer type.

Quick conclusion :

There can be many such examples when the wrongly specified product and the customer's strict standards not allowing deviations to the specifications end up with a wrong switch at the site.

The switches are simple to adjust, easy to install, and cost-saving devices but the selection criteria should be accurate and the vendors should be given a chance to express their suggestions. But, replacing switches with a costlier smart transmitter merely for a switching action, with expensive analog input and programming logic, and waiting for months and years to get the "Management of Change" Approval is not advisable.

World-leading companies like United Electric Controls, offer SIL 2 certified SIL 3 capable pressure, Diff. Pressure and temperature switches with a choice of micro switches solid-state electronics and a huge display make better sense.