For cut-off valve feedback, use a proximity switch
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Code for Selection and Design of Automation Instruments in Petrochemical Industries 10.3.6.13 Limit switches should preferably be of the flameproof proximity dry-contact type or NUMAR type; their explosion protection rating shall correspond to the classification level of the hazardous area, and their protection rating shall not be lower than IP65. Question 1: Is the term \"proximal dry contact type\" contradictory? A proximity limit switch is either NPN or PNP, right? Only mechanical proximity switches have dry contacts, don’t they? 2. Why don’t the standards recommend mechanical types? Most of the limit switches used on-site are still mechanical, right?
1. Your understanding is correct; a proximity switch generally refers to a sensor that operates electronically (using NPN or PNP outputs) and does not have dry contacts. Dry contacts generally refer to mechanical limit switches, which use physical contact to close or open a circuit. 2. Standards may recommend the use of flameproof proximity dry-contact limit switches, as in hazardous environments such as those in the petrochemical industry, this type of switch ensures safe and reliable operation thanks to its flameproof properties and high protection rating (IP65 or higher). Although mechanical limit switches are widely used, issues such as mechanical wear and dust accumulation may affect their long-term stability and safety. .
There are those with proximity dry contacts, such as TOPWORX’s 73 series
What dry contacts? It’s a word game; I don’t know. My understanding is that one is a mechanical contact, while the other is an electronic contact without any mechanical components. Mechanical types must be of the flameproof type, as contact can generate sparks. However, the DI inputs on DCS cards are well-designed to limit the current they can output; for example, the maximum current is only 10 mA, which basically prevents sparks from being generated that could ignite acetylene. Electronic types such as Hall effect sensors can be turned on or off under the influence of a magnetic field; they are also considered proximity switches, with it being the change in capacitance that causes the circuit to be turned on or off. These all lack physical contacts, so no sparks are generated when current is turned on from off; however, it is also necessary to consider whether the electronic components in the circuit can produce sparks.
From my experience, mechanical types have too high a failure rate; we only buy NAMUR types
This post was last edited by Suibing on 2024-6-29 09:00. Everyone says NAMUR is good, but I just ordered a proximity DPDT type device – it’s extremely expensive and not in stock.
I have Namur (24-8V) models here; they are indeed quite expensive, but I have dozens of them, so I can offer a lower price
If you are interested, you can call 13508967128