What this sector has to measure
A chemical plant punishes instruments with corrosion, crystallisation, fouling and temperature, so the specification ends up being written around the medium rather than around the measurement. The wetted-material and sealing decision usually matters more than the accuracy grade, because an instrument of modest accuracy with the right wetted face keeps working, while a precise one with the wrong one fails within weeks.
The sector also splits along a line that has nothing to do with chemistry: batch against continuous. A batch reactor asks for fast response and repeatable dosing, because the quantity added in a short window is the figure that matters. A continuous column or pipeline asks for drift stability over months, because nobody will recalibrate a unit that runs continuously and the accumulated error is what eventually shows up in the site balance.
Most of the difficulty arrives before the instrument is chosen. A medium that crystallises, a vapour space that condenses, a reagent that requires a lined pipe, a seal that has to isolate the transmitter from the process entirely - each of those is a decision about the installation, and each one changes what can be specified. An enquiry that states the medium, the concentration and how it behaves is answered far more precisely than one that names the measurement alone.
Where the instruments sit
Reaction and synthesis
Reactors, autoclaves and crystallisers, where temperature and pressure are the controlled variables and agitation makes level and interface measurement difficult. Corrosion and crystallisation are routine, and the instrument is often mounted through a nozzle where access is limited.
Separation and purification
Columns, evaporators, extractors and filters. Differential pressure across packed beds and trays is a recurring duty, and the composition changes from stage to stage, so one material specification rarely covers a whole unit.
Storage, transfer and dosing
Tanks, day tanks and dosing skids. Level and flow dominate, and the quantities are frequently small and repeated, which places the emphasis on repeatability rather than on a wide range.
Utilities and effluent
Cooling water, steam, nitrogen and the effluent treatment that closes the site. pH, conductivity, flow and level are measured here under a regulatory obligation as well as a process one, so the record matters as much as the reading.
Instrumentation used in this sector
| Measurement | What is measured in this sector | Instruments commonly specified |
| Flow | Reagent dosing, corrosive liquids and slurries, solvent transfer, cooling water and steam | Electromagnetic meters with the lining matched to the medium, Coriolis meters for dosing and mass, vortex or differential-pressure installations for steam, and positive-displacement meters for viscous and dosing duties |
| Pressure | Reactor and column pressure, vapour pressure, and differential pressure across beds, filters and strainers | Pressure and differential transmitters with diaphragm seals and capillaries so that the transmitter never sees the process, and with the fill fluid chosen for the temperature at the tapping |
| Temperature | Reaction temperature, column and condenser profiles, and the temperature used to compensate flow and analysis readings | RTDs in thermowells where stability is wanted, thermocouples where the duty is hot and fast, and surface or non-contact measurement for vessels that cannot be tapped |
| Level and interface | Tank inventory, reactor level under agitation and foam, and the boundary between two immiscible liquids | Radar and guided-wave transmitters, hydrostatic transmitters where the vessel is simple, interface instruments where a phase boundary is the reported figure, and independent switches for protection |
| Analysis | pH, ORP, conductivity, dissolved oxygen and moisture in reaction, utility and effluent streams | In-line analysers and transmitters with the electrode or cell selected for the chemistry, plus the sample conditioning that has to sit in front of them |
| Gas and vapour | Combustible gas, toxic gas, oxygen deficiency and vapour concentration in process areas, tank farms and dosing rooms | Fixed detectors with the sensor type matched to the target gas, and portable instruments for entry and maintenance work |
The list is a starting point rather than a quotation. In this sector the wetted material, the seal arrangement and the area classification narrow the choice faster than the measurement itself does.
How to specify for a chemical plant
- Wetted materials as a project standard rather than a preference. Lined bodies, high-alloy wetted parts and ceramic faces are specified because a particular medium demands them, so the material standard is the first thing to send.
- Whether the transmitter may see the process at all. Where it may not, a diaphragm seal and capillary become part of the measurement rather than an accessory, and the fill fluid, the capillary length and the temperature each affect the result.
- Crystallisation, polymerisation and solids. A medium that deposits will coat a wetted face and change the reading, so the requirement is often for a flush or non-contact design rather than for a finer accuracy class.
- Batch or continuous, because the two make opposite demands on response and drift and one instrument rarely satisfies both well.
- The area classification and the protection concept, which decide which instruments are eligible and how the installation is carried out - frequently within the same plant.
- Whether the medium is hazardous to the environment as well as to people, since a containment requirement can impose a design and a material specification that a process requirement does not.
- Materials for the whole pressure boundary, not only the wetted face. The flange, the bolts and the gasket are part of it and are often attacked by the same atmosphere that attacks the process side.
- Which measurements feed the batch record or the effluent report, because those carry a stability and documentation requirement that a control reading does not.
Standards and approvals that may apply
- Pressure equipment and piping material standards applied by the project, commonly expressed as a piping class or a material selection standard.
- Hazardous-area certification schemes, which the installation may be required to carry. We confirm which approval a selected model holds, and we state it as unconfirmed where the documentation has not been supplied to us.
- Hygienic or clean-in-place requirements, which appear in chemical plants that also produce a product for human contact.
- Environmental discharge and emission monitoring obligations, where a measured value is reported to an authority and the method is fixed rather than chosen.
- We describe what an installation may have to meet. We do not claim that any approval is held for a product unless the certificate has been supplied to us, and the product pages follow the same rule.
Applications in this sector
