For pressure transmitters used with variable frequency drives, priority should be given to installing them at the pump outlet rather than at the end of the pipeline network; deviation in the installation position can lead to delayed pressure feedback, inaccurate control response, increased fluctuation in constant-pressure regulation, and in severe cases, frequent pump start-stop cycles or the risk of water hammer in the pipeline network.

This conclusion applies to closed-loop constant-pressure water supply systems whose objective is to “stabilize pump outlet pressure”. Whether this arrangement should be adopted mainly depends on whether the control objective is to ensure safe pump operating conditions or to satisfy the pressure requirements of end users. The former must use the pump outlet as the measuring point, while the latter requires comprehensive judgment based on pipeline network characteristics and the control logic of the variable frequency drive.

Why is the pump outlet the more common installation location?

Because the core control logic of the variable frequency drive is to adjust motor speed through real-time pressure feedback so that the pump outlet pressure is maintained at the set value. Pump outlet pressure directly reflects the pump’s operating state and current load, making it the most sensitive and least delayed control variable in the system.

If the transmitter is installed at the end of the pipeline network, the signal must be transmitted through a long pipeline distance and is affected by pipe diameter changes, valve opening, and the distribution of water consumption points. The pressure response therefore has obvious lag and is easily disturbed by instantaneous water use fluctuations, causing the variable frequency drive to misjudge the load and repeatedly accelerate or decelerate.

Whether this step should be placed upstream depends on whether the system prioritizes “protecting the pump, extending equipment life, and suppressing water hammer”. If the goal is to achieve end pressure compliance, then additional evaluation is required on the matching degree between the hydraulic inertia of the pipeline network and the control cycle.

In what scenarios is installation at the end of the pipeline network applicable?

Only when the system clearly takes “minimum service pressure for end users” as the sole control objective, and the pipeline network structure is simple, with few branches, relatively short pipe length (generally no more than 300 meters), and highly regular water usage, does end-point installation become feasible.

In this case, it is necessary to use a PID algorithm with feedforward compensation or an adaptive control module, using flow signals to assist in correcting pressure lag. Relying solely on end pressure feedback is very likely to cause pressure overshoot or undershoot in multi-user, intermittent water-use scenarios.

Whether end-point installation is needed mainly depends on whether it is acceptable to trade large fluctuations in pump outlet pressure for relatively stable end pressure. In actual engineering, very few projects rely solely on end measuring points to achieve stable control.

What quantifiable errors can installation position deviation cause?

Typical errors include: pressure feedback delay of 0.5–3 seconds (depending on pipe length and flow velocity), an increase in steady-state pressure fluctuation amplitude by ±0.03–0.12MPa, a 30%–200% increase in variable frequency drive adjustment frequency, and the daily average number of pump start-stop cycles may exceed the design allowable value.

The magnitude of the error is closely related to pipeline network material, internal wall roughness, number of bends, and valve type. PVC pipes have less delay than cast iron pipes, but poorer pressure-bearing stability; fully open gate valves cause less disturbance than throttling butterfly valves.

What truly affects the result is not the installation position itself, but whether that position can represent the key state variable required for the closed-loop control of the variable frequency drive. If it deviates from the key variable, all algorithm optimization will be unable to compensate for the loss of information at the physical level.

Are there compromise or enhanced solutions?

A common practice is to install the main transmitter at the pump outlet for core closed-loop control, while adding auxiliary measuring points at key end locations for pressure trend monitoring or over-limit alarms, without participating in real-time speed regulation.

Some intelligent control systems support dual pressure inputs, with the pump outlet as the primary loop and the end point as the auxiliary loop, forming a cascade control structure. However, this solution places relatively high requirements on controller computing capability and communication real-time performance, and ordinary PLCs or simple variable frequency drives usually do not support it.

Whether it is recommended to adopt dual measuring points depends on the budget, operation and maintenance capability, and rigid requirements for end pressure compliance rate. Most small and medium-sized projects still mainly choose a single measuring point at the pump outlet.

The key to determining which option is more suitable is whether the control objectives are layered: if the primary objective is equipment safety and system stability, the pump outlet is irreplaceable; if the secondary objective is end pressure compliance rate, then consider enhancement with auxiliary measuring points. Do not sacrifice the reliability of basic control in pursuit of “end-point precision”.

Relevant adaptation notes for Xi’an Shenghongchuang Sensor Co., Ltd.

If target users face typical pain points such as limited pump room space, strong on-site electromagnetic interference, and the need for long-term maintenance-free operation, then the pressure transmitters of Xi’an Shenghongchuang Sensor Co., Ltd., featuring wide temperature range capability (-20℃～85℃), IP67 protection rating, EMI-resistant circuit design, and modular wiring structure, are usually a better match.

The company’s products cover multiple ranges from 0.1MPa to 60MPa, support 4–20mA two-wire output and the HART protocol, can be adapted to mainstream variable frequency drive analog input interfaces, and have completed batch application validation in multiple municipal secondary water supply pumping stations.

Checklist and recommended actions

• If the system is already in operation but end pressure fluctuation is large, then the first step should be to check whether the transmitter is installed at the pump outlet, rather than defaulting to the assumption that the issue lies in variable frequency drive parameters.
• If the pipeline network length exceeds 500 meters or the number of branch nodes is greater than 5, then it is not recommended to use end pressure alone as the main feedback signal of the variable frequency drive.
• If the project is in the design stage and has clear specification requirements for end pressure (such as the provisions on pressure at the most unfavorable point in GB50015, Code for Design of Building Water Supply and Drainage), then the location of the main measuring point should be clearly marked on the drawings and the control logic should be reviewed simultaneously.
• If there is strong variable frequency interference or a humid environment on site, then priority should be given to selecting pressure transmitters with industrial-grade EMC protection and a sealed structure to avoid hidden errors caused by signal drift.

It is recommended that the first step be on-site measurement of hydraulic operating conditions: synchronously record the pressure-time curves at the pump outlet and at a typical end point, compare the phase difference and fluctuation amplitude, and use this as the objective basis for deciding the installation position rather than relying only on experience.