How To Install Pressure Tapping Points & Impulse Piping In Pilot Plants For Accurate Measurement?

Precise pressure measurement in a pilot plant begins not with the transmitter, but with the installation of the tapping point and impulse line. To ensure accurate pressure readings, the tap must be located in a straight, stable-flow section of pipe, oriented correctly for the fluid phase, and connected via a short, sloped impulse line of appropriate diameter. The most expensive transmitter will yield useless data if its impulse lines are clogged with condensate, filled with gas bubbles, or subjected to turbulence from a nearby elbow.

The core challenge is not the instrument itself, but the physical connection between the process and the sensor. A poorly placed tap or an incorrectly routed impulse line can introduce errors that dwarf the transmitter’s inherent accuracy. The guidelines below—tapping point location, proper phase-aware orientation, and careful impulse line design—are the non-negotiable foundation for trustworthy data.

The Foundation: Selecting the Right Tapping Point Location

Pressure is only meaningful if it reflects the true static pressure of the fluid. Any disturbance in the flow path can create local pressure variations that corrupt the measurement.

Avoid Turbulence: Straight Pipe is Non-Negotiable

The tapping point must be in a straight pipe section with fully developed, stable flow. Avoid locations immediately downstream of bends, valves, tees, or other fittings that generate swirls and eddies. In pilot plants, where space is tight, it is tempting to place the tap on an elbow for convenience—this guarantees inaccurate, non-reproducible readings.

The Influence of Pipe Diameter and Burr-Free Installation

In the small-diameter lines common in pilot plants, even a minor internal protrusion from the tap hole can disrupt the flow pattern. The tap hole must be drilled flush with the pipe inner wall and thoroughly deburred. While the supplementary straight-run requirements for flowmeters (10 pipe diameters upstream) are not directly mandated for pressure taps, applying a similar principle—at least 5–10 straight diameters upstream—is a conservative best practice that promotes a stable velocity profile at the measurement point.

Phase-Specific Tapping Orientation and Transmitter Mounting

The physical state of the process fluid dictates where on the pipe circumference the tap should be placed and where the transmitter is mounted relative to the tap. Getting this wrong is one of the most common root causes of measurement drift and failure.

Liquid Service: Keep Gas Out

For liquid lines, any trapped gas pocket acts as a compressible buffer that delays and distorts the pressure signal. Mount the tap on the side or, ideally, in the lower half of the pipe to prevent gas entrapment. More critically, mount the transmitter below the tapping point. This lets any entrained gas rise back into the process line while the impulse line remains liquid-filled, guaranteeing a solid, incompressible pressure transmission path.

Gas Service: Keep Liquids Out

Condensation or entrained droplets in a gas line create a liquid slug that can block the impulse line or add a variable hydrostatic head error. Place the tap in the upper half of the pipe to avoid drawing liquid into the impulse line. The transmitter should be mounted above the tap point so that any liquid that does enter drains back into the process by gravity.

Steam and High-Temperature Fluids: The Condensing Loop

Sending live steam or hot condensate directly into a pressure transmitter will instantly damage the sensor element. A condensing loop, also known as a siphon, must be installed between the tap and the transmitter. This water seal fills with cool liquid, protecting the instrument while still faithfully transmitting pressure. For vertical impulse lines, a pigtail siphon is typical; for horizontal runs, a simple U-shaped loop serves the same purpose.

Impulse Line Design: The Conduit for Pressure Signals

The impulse line is the hydraulic pathway that carries the process pressure to the transmitter. Its geometry directly controls response time, clogging risk, and measurement stability.

Sizing the Line: The 6-10mm Rule

The internal diameter should be between 6 and 10mm. A smaller ID increases the risk of plugging from particulates or viscous fluids. A larger ID works hydraulically but is heavier, harder to route, and can amplify dynamic effects like vibration or thermal expansion. For clean pilot plant services, 6mm tubing is often ideal; for fluids that may contain some solids, lean toward 10mm.

Slope and Routing: Let Gravity Help You

Every impulse line must be sloped continuously toward either the process tap or the transmitter—never flat and never with a low trap. The recommended slope is 1:10 to 1:20 (approximately 5% to 10% grade). This allows gases in a liquid line to rise to the process and liquids in a gas line to drain back. In pilot plants, where lines are frequently modified, maintaining this slope is easily overlooked, yet it is the single most powerful defense against measurement errors caused by phase separation.

The 50-Meter Limit: Why Shorter is Better

Impulse lines should be kept as short as possible, ideally under 50 meters. Long impulse lines increase frictional damping, slow the transient response, and provide more surface area for heat loss or chemical degradation. In the confined spaces of a pilot plant, excess tubing coiled for convenience introduces unnecessary volume that degrades measurement fidelity. If distance is unavoidable, consider using a diaphragm seal with a filled capillary to eliminate the impulse line entirely.

Trade-offs and Common Pitfalls in Pilot Plant Pressure Installations

Even when the basic rules are followed, real-world constraints force compromises. Understanding these trade-offs is what separates reliable data from persistent troubleshooting.

The Dilemma of Long Lines vs. Response Time

Short lines are ideal, but safety or accessibility may demand remote mounting. Every extra meter of impulse line slows the sensor’s response to pressure changes. For control loops, this added lag can cause instability. When long runs are forced, use a slightly larger tubing (e.g., 12mm) to reduce frictional resistance, but be aware this makes the system more susceptible to temperature-induced errors.

When Slopes Become a Problem in Cramped Spaces

Achieving a 1:10 slope in a dense, multi-tier pilot plant rack is often physically impossible without eating up precious vertical clearance. A common workaround is to use a diaphragm seal with a capillary fill to replace the impulse tubing entirely. This solution eliminates slope concerns but introduces a higher cost and a small temperature sensitivity of the fill fluid.

The Hidden Enemy: Vibration and Thermal Effects

Pilot plants are filled with pumps, compressors, and stirrers that generate high-frequency vibration. If the impulse line or transmitter bracket is not rigidly anchored, vibration couples directly into the sensor, creating a noisy, drifting signal. Additionally, uneven thermal expansion between a long impulse line and its support can impose mechanical stress on the transmitter connection, leading to zero drift over time. Always use robust brackets and allow for slight thermal flexibility.

How to Apply This to Your Pilot Plant

The optimal installation strategy depends on your dominant process fluid and the constraints of your facility. Use the following goal-oriented guidelines to make the right call.

If your primary focus is high-accuracy liquid pressure measurement: Mount the transmitter below the lowest possible tapping point, slope the impulse line continuously downward toward the transmitter, and eliminate any high points where gas could collect.
If your primary focus is gas service with variable humidity or condensing vapors: Mount the transmitter above the tap, slope the line continuously upward toward the process, and consider a small condensate pot at the transmitter inlet if liquid slugs are a recurring problem.
If your primary focus is steam or hot oil above 120°C: Never omit the condensing siphon or liquid seal. Insulate the siphon and impulse line to maintain a stable fill temperature and prevent false readings from partial vaporization.
If your pilot plant layout forces extremely tight bends or inaccessible tap locations: Replace impulse lines with a remote diaphragm seal and filled capillary system. This eliminates slope and clogging risks at the cost of higher initial investment and a minor temperature coefficient.

By mastering these seemingly mundane installation details, you transform your pressure data from a chronic source of error into a rock-solid foundation for scale-up and process insight.

Summary Table:

Service Type	Tap Orientation	Transmitter Position	Key Design Guideline
Liquid	Side or lower half	Below tapping point	Prevent gas entrapment; slope line 1:10 to 1:20
Gas	Upper half	Above tapping point	Avoid liquid accumulation; slope line 1:10 to 1:20
Steam / Hot Fluid	Phase-dependent	Via condensing loop/siphon	Use a siphon/pigtail loop to protect sensor from heat

Achieve Measurement Accuracy with LABPARK Pilot Plants

Accurate data collection relies on properly designed systems. LABPARK provides state-of-the-art Educational and Vocational Unit Operations Pilot Plants in chemical engineering, bioprocess & biotech, and environmental & water treatment for universities, research institutes, and enterprises.

Let us help you design and equip your laboratory with reliable, industry-standard systems. Contact our team today to find the perfect pilot plant solution for your institution.