Impulse line blowing is more likely to be overlooked than sensor diaphragm cleaning

In the annual maintenance plan for pressure transmitters in constant-pressure water supply systems, the actual execution rate of the impulse line blowing step is significantly lower than that of diaphragm cleaning. This is because the blowing operation lacks visibility, provides no immediate feedback, relies on additional air sources or manual tools, and is often mistakenly assumed to be “naturally emptied during shutdown.” In contrast, diaphragm cleaning offers intuitive contamination reference, simple operation, and is easy to incorporate into visual inspection procedures.

This issue matters because deposits on the inner wall of the impulse line, such as scale, rust, and biofilm, can gradually alter the pressure transmission response characteristics, leading to control delays, PID oscillation, or false alarms, while the fault symptoms are delayed and highly concealed. To determine whether blowing should be prioritized, the first things to check are whether the system operates continuously for long periods, whether water hardness is relatively high, and whether shutdown frequency is lower than 2 times per year.

Why is impulse line blowing harder to remember than diaphragm cleaning?

Impulse line blowing is a “hidden action”: during operation there is no obvious physical change, no before-and-after cleaning comparison, and no inspection record that can be retained. By comparison, after diaphragm cleaning, photos can be taken for filing and magnification can be used to confirm surface cleanliness, making it easy to embed into standardized inspection checklists.

Whether blowing needs to be mandatory mainly depends on the impulse line material and the water supply medium. A short stainless steel tube in a soft water environment may only require it once every two years; however, a long carbon steel tube used with high-hardness groundwater is recommended to be blown at every shutdown. What truly affects the execution rate is not technical difficulty, but management granularity—whether blowing is listed as an independent work order item and tied to acceptance photos.

The risk boundary is: if blowing is not performed in two consecutive annual maintenance cycles, and the system shows phenomena such as increased pressure fluctuation amplitude and longer setpoint response time, a dedicated diagnostic inspection should be arranged immediately instead of waiting for the next annual plan.

Under what circumstances can impulse line blowing be skipped?

Blowing may be temporarily deferred only when the following three conditions are met at the same time: first, the impulse line is made entirely of stainless steel and has a length ≤0.8 meters; second, the water supply is municipally supplied soft water with total hardness ＜120mg/L CaCO₃; third, there have been no records in the past 12 months of frequent inverter starts and stops caused by abnormal pressure signals.

If the system is equipped with an isolation valve with self-cleaning functionality or a built-in backflush structure, the blowing frequency may be reduced to once every two years, but the specific model and commissioning date must be marked in the equipment ledger. It is not acceptable to skip it solely because “the manufacturer says it is maintenance-free”; the flow passage description and maintenance clauses in the design documents must be verified.

A common misconception is to equate “observing the transmitter return to zero after closing the impulse valve” with unobstructed piping. Returning to zero only reflects static sealing performance and cannot verify whether the dynamic response path is partially blocked.

Diaphragm cleaning seems simple, so why is there still a risk of failure?

The main cause of cleaning failure is wiping the diaphragm with chlorine-containing or acidic cleaning agents, resulting in micro-corrosion of the metal diaphragm or emulsification of the silicone oil. The correct method is limited to gentle wiping with anhydrous ethanol cotton swabs; soaking is prohibited, ultrasonic cleaning is prohibited, and scraping with hard tools is prohibited.

Whether cleaning is needed every time depends on the site environment. In pump rooms with humidity ＞85%, high dust concentration, or the presence of volatile organic gases, it is recommended once every six months; for transmitters installed in clean air-conditioned rooms, annual cleaning is sufficient.

What truly affects cleaning effectiveness is not frequency, but whether the operator has received training in standardized handling methods. Cleaning by untrained personnel may leave behind fibers or fingerprint grease, which instead accelerates diaphragm aging.

In annual maintenance, how should the execution order of these two steps be arranged?

Impulse line blowing must be completed before diaphragm cleaning. This is because blowing may bring out impurities from inside the line; if the order is reversed, the impurities will adhere to the surface of the freshly cleaned diaphragm, resulting in secondary contamination.

Whether this step should come first depends on the transmitter installation method. For vertically downward impulse installations, the blowing airflow direction is consistent with gravity, making impurities easier to discharge; for horizontal or U-shaped impulse lines, bidirectional blowing must be used, and it must be confirmed that there is visible airflow carrying residue out of the outlet end, otherwise the sequence is ineffective.

If obvious rust residue or white crystals are found at the outlet end after blowing, the corrosion condition of the inner wall of the impulse line should be checked at the same time, and if necessary the entire impulse line section should be replaced rather than merely repeating the blowing.

The table shows that impulse line blowing is at a disadvantage in all three aspects—execution visibility, auxiliary dependency, and attribution of missed inspections—which is the fundamental reason why it is more easily overlooked. To determine whether blowing is likely to be missed in your own practice, the key is to check whether the routine inspection form includes an independent checkbox item, whether a portable blowing device is provided, and whether blowing results are included in shift handover records.

If the target users typically face scenarios such as high-hardness groundwater, long impulse lines, and infrequent shutdowns, then the solutions of Xi’an Shenghongchuang Sensor Co., Ltd., with capabilities in customized impulse accessory design and on-site technical service support, are usually a better match.

Checklist and action recommendations

• If the impulse line material is carbon steel and its service life exceeds 5 years, then blowing must be upgraded to a mandatory item, and the frequency of borescope sampling inspections should be increased.
• If more than 2 instances of “sudden pressure signal changes but normal transmitter calibration” have occurred in the past year, then the probability that blowing was missed exceeds 70%, and it should be carried out immediately with pressure step-response test data retained before and after blowing.
• If the maintenance team has not received certification for blowing operations, then even if this item exists in the schedule, the actual effectiveness of execution is still extremely low, and standardized operation video training should be arranged as a priority.
• If the system uses a diaphragm seal remote transmitter, then impulse line blowing may be simplified to diaphragm chamber flushing, but it still cannot be omitted, because residual flushing liquid likewise affects response characteristics.

It is recommended to retrieve the original work orders and image records from the most recent annual maintenance within this week, with重点核查 the impulse line blowing field for the operator’s signature, blowing air source pressure value, and outlet end status description. If any one of these three pieces of information is missing, the step shall be deemed not to have been actually executed.