Technology Zone
Pressure control and measurement protect pipelines, vessels and pumps from running outside their limits. This zone covers transmitters, gauges, switches, regulators and relief devices.
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What this zone covers
Three references, and mixing them up is the most common error on a datasheet. Gauge pressure is measured against the atmosphere and is what a pump discharge or a vessel normally quotes. Absolute pressure is measured against vacuum and is what vacuum systems, boiling points and gas calculations need. Differential is the difference between two taps and is what flow elements, filters and level in closed vessels rely on.
Nearly all industrial transmitters put a thin diaphragm between the process and a measuring element. A piezoresistive cell reads the strain as a resistance change and is compact, fast and cheap; a capacitive cell reads the movement between two plates and is more stable over years and over temperature. Both are protected by an isolating diaphragm and a fill fluid, and both live or die by how that diaphragm is treated.
Control is not the same as protection. A regulator holds a downstream pressure by throttling itself, quietly and without a control loop, which is what instrument air, gas blanketing and utility lines use. A relief valve or a rupture disc does nothing at all until the set pressure is reached and then opens fully. Every vessel that can be blocked in needs the second one, and no amount of control logic replaces it.
The transmitter is usually right; the impulse line is usually not. Condensate in a gas leg, gas in a liquid leg, a blocked tap or a frozen line all shift the reading without any alarm. Chemical seals with capillaries put the diaphragm directly on the process and remove the line altogether, at the cost of a temperature effect from the fill fluid and a long list of installation rules about height and capillary length.
Pressure is the cheapest diagnostic in the plant. Differential across a filter tells you when to change it, across a heat exchanger when to clean it, and across a pump whether the impeller still delivers head. Trending three differential readings usually finds more problems than a full vibration programme, because the instruments are already there.
| Parameter | Typical range | Rule of thumb |
|---|---|---|
| Turndown of a transmitter | Calibrate within 10:1 of the upper range limit | A cell used far below its range multiplies its own drift and temperature error |
| Accuracy | 0.075 to 0.2 % of span for a process transmitter | Add the ambient temperature effect; over a year that often exceeds the base accuracy |
| Overpressure limit | Two to ten times the upper range value | Size on the worst upset, including pump shut-off head and hydraulic shock |
| Diaphragm material | 316L as standard, Hastelloy or tantalum for aggressive duty | The wetted diaphragm is thin; corrosion that a pipe shrugs off will hole it |
| Relief valve set pressure | At or below the design pressure of the vessel | Accumulation of 10 % is normal; check the vessel can take set plus accumulation |
| Regulator droop | 5 to 20 % between no flow and full flow | Size on the flow range, not on the maximum; an oversized regulator hunts |
| Capillary length, chemical seal | Under 3 m, matched on both legs | Long or unequal capillaries turn ambient temperature into a false differential |
| Response time | 100 to 500 ms including damping | Damping smooths the trend and hides the surge you were trying to catch |
| Symptom | Likely causes | What to do |
|---|---|---|
| Reading drifts up over months | Zero shift from overpressure, mounting strain, or a slow change in the fill fluid | Vent both sides and check the zero at atmosphere; re-zero after any overpressure event |
| Differential reading is negative or nonsense | Impulse legs swapped, condensate in the dry leg, or gas trapped in a wet leg | Blow through both legs, confirm which tap is high, and slope the lines so they drain the right way |
| Signal is noisy on a pump discharge | Pulsation from a positive displacement pump or a partly closed valve nearby | Add a snubber or a small amount of damping, and move the tap away from the valve or the elbow |
| Transmitter reads correctly cold, wrong when hot | Ambient temperature effect on a chemical seal, unmatched capillaries, or direct sun on one leg | Shade both capillaries equally, keep them the same length, and check the temperature coefficient in the datasheet |
| Relief valve leaks below set pressure | Operating pressure too close to the set point, seat damage, or back pressure from a shared header | Keep the normal operating pressure at 90 % of set or below, lap or renew the seat, and check the header design |
| Regulator hunts or sings | Oversized for the actual flow, or the downstream volume is too small to damp it | Resize on the real flow range and add volume or a restrictor downstream |
| Filter differential rises the moment it is cleaned | Element fitted the wrong way round, bypass valve left open, or a blocked impulse tap rather than a blocked filter | Confirm the taps read the same with the filter open, then check the element and the bypass |
Go deeper: pressure control and measurement news and case studies · frequently asked questions
Equipment Guide
Endress+Hauser AG
Endress+Hauser is a global leader in measurement instrumentation, services and solutions for industrial process engineering. We provide process solutions for flow, level, pressure, analytics, temperature, recording and digital communications, optimizing processes in terms of economic efficiency, safety & environmental impact.
Siemens Industry, Inc.
Siemens Process Instrumentation offers innovative measurement solutions to increase plant efficiency and enhance product quality. Siemens extensive portfolio of process instruments for flow, level, pressure and temperature measurement, provides critical data that enables you to optimize your process, reduce energy consumption and ensure safety.
Brooks Instrument
For over 75 years, Brooks Instrument has been a trusted partner because our flow, pressure and vaporization instrumentation is more than accurate. It is reliable. Stable. Repeatable. Durable. It’s proven year after year, decade after decade, in the most demanding industrial and electronic manufacturing systems.
GF
GF is the leading flow solutions provider worldwide, enabling the safe and sustainable transport of fluids. The company specializes in plastic piping systems and system solutions plus services in all project phases.
Bronkhorst High-Tech B.V.
Bronkhorst High-Tech offers an extensive product range of thermal, Coriolis and ultrasonic flow meters and controllers. We offer numerous styles of both standard and bespoke instruments for applications in laboratory, machinery and industry and we closely cooperate with OEM’s in the field of customer specific designs for gas, liquid or vapour control.
Anderson-Negele
Anderson-Negele is a global company specializing in the development and production of sensors and measuring equipment for hygienic applications. As your reliable and flexible partner, we aim to always provide you with the best solution for your process.
Ask the Expert
Engineers and product specialists who answer technical questions from readers. The answer comes back to you by e-mail; when we publish it, other readers benefit too.
Jan ChristensenBrooks Instrument
Jan Christensen has nearly 40 years of experience in precision flow and pressure control instrumentation. He began his career as a Canadian… Ask a question
Natalie WaldeckerVEGA Grieshaber KG
Natalie Waldecker, Portfolio Manager Food and Pharma, knows the challenges of the demanding industries inside out. With her broad application knowledge, she… Ask a question
Ryan FitzgeraldAnderson-Negele
Ryan Fitzgerald, Product Manager at Anderson-Negele, is an expert for the process management sensor portfolio which includes electromagnetic flow meters, Coriolis flow… Ask a question
Erik TiemensmaBronkhorst High-Tech B.V.
Erik Tiemensma, Sales Knowledge Manager, has over 20 years of experience in flow and pressure measurement and control for gas, liquid and… Ask a question
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What this zone covers
Use gauge where the atmosphere is part of the system, which covers pumps, vessels and most utility lines. Use absolute wherever the answer must not move with the weather: vacuum systems, boiling points, gas density calculations and anything that feeds a flow computer. On a low range instrument the difference between the two is far larger than the accuracy you are paying for.
An overpressure event stretches the isolating diaphragm slightly, and the zero moves with it. The span usually survives, so the trend still looks plausible, which is what makes it dangerous. Re-zero at atmosphere after any known upset, and choose an overpressure rating based on pump shut-off head and hydraulic shock rather than on the normal duty.
When the impulse line is the problem: viscous, crystallising, freezing or corrosive products, or a tap that has to be flushed to stay open. The seal puts the diaphragm on the process and removes the line altogether. The price is a temperature effect from the fill fluid, so keep the capillaries short, equal in length and shaded, or that effect becomes the biggest error in the loop.
No. A controller depends on power, on signal, on a valve that still moves, and on the fault not being in the logic itself. A relief valve or a rupture disc is mechanical, needs nothing, and opens on the physics. Every vessel that can be blocked in gets one, and the control loop is what keeps you from using it.
About ten to one from the upper range limit before the numbers start to disappoint. The base accuracy is quoted on span, but drift and temperature effects are tied to the cell, so a transmitter calibrated to a twentieth of its range carries twenty times the relative error. Buying a smaller cell is nearly always cheaper than living with the error.
On gas, take off the top so condensate drains back; on liquid, take off the side so gas does not collect; on steam, use a condensate pot and let the leg fill. Keep the tap away from elbows, valves and pumps by at least a couple of diameters. Most pressure problems are decided by these three choices before the instrument is even unpacked.
How fouled something is, for almost no money. Across a filter it sets the change-out point, across a heat exchanger it shows scaling long before the duty falls away, and across a pump it tells you whether the impeller still produces its head. The instruments are usually already installed; what is missing is someone trending the three numbers together.
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