Two Common Problems of Pressure Vessels in Air Compressor Systems – Does Your Facility Have These Hidden Risks?

As an important energy source, air compressors are widely applied in all links of production and daily life. Many electronics manufacturers in Wujiang extensively use compressed air on their production lines.
Most air compressor systems in enterprises are equipped with air/oil separators and air receivers. Both pieces of equipment fall under pressure vessels, which require usage registration and periodic inspection.
Pressure vessels are special equipment with significant hazards. Their original operating conditions shall not be altered arbitrarily. Please check whether your enterprise has the following two common violations:
Random sealing of flange connections with blind plates during pipeline modification

Standard blind flanges must be adopted for pressure vessel flanges, as standard blind flanges are designed with sufficient strength, rigidity and other performance requirements. 

Nevertheless, operators often ignore this requirement and seal flange outlets arbitrarily, inevitably bringing potential safety hazards.

Case

Recently, inspectors from the Pressure Equipment Section, Wujiang Branch conducted an on-site inspection for an operator. They found that non-standard blind plates were used to seal pipe flanges of pressure vessels, 

and fastening bolts were not fully installed, which seriously violated design specifications. Inspectors immediately issued rectification requirements for the enterprise and eliminated a potential accident hazard.

Misconception: The system can operate without safety valves due to safety interlock protection devices

As overpressure relief devices, safety valves play an indispensable role in safe operation of pressure vessels. When the vessel exceeds rated pressure, 

interlock devices cannot directly and rapidly discharge internal pressure, which may trigger overpressure explosion of the vessel. Therefore, safety interlock devices cannot replace pressure relief devices for pressure vessels.

Case

During a recent pressure vessel inspection at one enterprise, our inspectors discovered that the air receiver was not fitted with safety valves as required by technical documents. 

The original connecting port was directly sealed by a pipe plug. Operation under overpressure conditions would lead to severe risks.

Friendly Reminder

If the pressure vessel can only be shut down temporarily due to production demands, the operator may prepare a spare safety valve of identical model and specification. 

Complete calibration first, then carry out replacement to shorten downtime.

Real Data – Revealing the Untold Secrets of the Combined Dryer Industry

The two mainstream compressed air dryers widely adopted in the industry are refrigerated compressed air dryers and desiccant air dryers. Refrigerated air dryers feature zero purge air consumption and low energy consumption, 

yet they are limited by achievable dew point. Desiccant air dryers can deliver low dew points, but suffer from high purge air loss and higher power consumption.

Low-dew-point combined compressed air dryers integrate the respective advantages of refrigerated dryers and desiccant dryers. Via reasonable pipeline connection and capacity matching, 

the strengths of both units are maximized to reach the optimal economical operating point and supply high-quality compressed air with ultra-low dew point.

Working Principle
Simply put, a combined dryer integrates a conventional refrigerated air dryer and a desiccant air dryer.
Multiple process layouts exist within the industry. The most mature and common solution is series installation of refrigerated air dryer and desiccant air dryer.

Moist compressed air discharged from the air compressor: Pressure 7 Barg, Temperature ≤45°C, Flow rate 45 Nm³/min. The water content of compressed air reaches 65 g per cubic meter. 

The total water content of airflow leaving the compressor is 375 g/min, equivalent to 22.5 kg of moisture entering downstream equipment every hour, or 90 kg every four hours.

If a refrigerated air dryer is applied to dry such moist compressed air down to a dew point of 15°C, the volumetric water content will drop to 12 g/m³. 

That means the water content at the dryer outlet accounts for 12/65 = 18% of inlet moisture. The refrigerated dryer removes 82% of total moisture. 

The load of the subsequent desiccant dryer is reduced to only 18%. This drastically lowers the burden of the desiccant dryer, enabling better dew point performance, 

extending adsorption cycle duration, reducing regeneration frequency, and cutting energy consumption significantly.

Process Flow of Combined Dryers
Warning

One type of process design in the industry directly feeds low-temperature outlet air from the refrigerated dryer into the adsorption tower of the desiccant dryer. 

This layout forces the adsorption tower to operate under low temperature. If the upstream filter delivers poor water removal performance, liquid water mist can penetrate the adsorption tower and cause dew point fluctuation.

The typical phenomenon: the adsorption tower performs well most of the time, yet the dew point suddenly deteriorates and recovers rapidly at irregular intervals. This fault occurs when tiny water droplets penetrate the adsorption tower.
Water droplets have far higher density than compressed air, carrying greater momentum in airflow. The droplets generate repeated elastic collisions with adsorbents without full adsorption, and finally pass through the adsorption tower.

Therefore, moist compressed air entering the adsorption tower must be free of liquid water mist. High-performance pre-filters are required. In addition, 

reheat measures shall be adopted to ensure all liquid droplets fully vaporize into steam before entering the adsorption tower.

For this reason, we do not recommend directly feeding low-temperature compressed air from the evaporator of the refrigerated dryer into the desiccant dryer. 

In other words, the widely recognized rational solution is the so-called “combined dryer” with refrigerated dryer and desiccant dryer installed in series.

Previously, some designs attempted to recover heat generated by the refrigeration compressor of the refrigerated dryer to heat purge air for desiccant regeneration, aiming to reduce heater power of the desiccant dryer.

Theoretical calculations and practical operation verify that the heat generated by the refrigeration compressor is low-grade and insufficient in quantity, 

and cannot meet the regeneration requirements of the desiccant dryer.

Important note: To achieve a dew point of -60 ~ -70°C, molecular sieve must be adopted as part of the adsorbent inside the desiccant dryer. 

The regeneration temperature of molecular sieve must reach ≥180°C with continuous heating time of no less than 1.5 hours; otherwise, effective regeneration cannot be realized.

Therefore, utilizing waste heat from refrigeration compressors to heat regeneration air has neither technical value nor economic benefits.
For applications requiring -40°C dew point, activated alumina needs regeneration temperature higher than 80°C; heating to 120°C is generally reasonable and safe.

For desiccant dryers with long switching cycles, regeneration gas below 80°C cannot supply sufficient heat to desorb moisture. Hence, 

the design concept of short-cycle micro-heat desiccant dryers recommended by some manufacturers is flawed.

Distinction of the Two Process Layouts
Structural Characteristics
A combined dryer adopts a refrigerated air dryer and a desiccant air dryer mounted on an integrated base. The whole unit is skid-mounted and ready for floor installation.
Functions & Features
Energy-saving and High Efficiency

The energy consumption of the refrigerated dryer section is consistent with conventional refrigerated dryers. As the operation cycle of the desiccant dryer section is extended, 

its power consumption is only 1/2 ~ 1/3 of a traditional micro-heat desiccant dryer.

A properly configured combined dryer consumes approximately 6% ~ 7% of the air compressor’s power output to achieve -60 ~ -70°C dew point, far lower than standard micro-heat desiccant dryers.
Stable Dew Point
The load of the desiccant dryer is only around 20% of conventional operating conditions, making it easy to achieve a stable -70°C dew point.

It should be clarified that the volumetric flow of compressed air remains unchanged, so the diameter and height of adsorption towers cannot be reduced, 

and manufacturing costs cannot be lowered substantially. Nevertheless, achievable dew point performance is greatly improved.

Reliable Operation
Both the refrigerated dryer and desiccant dryer operate under favorable working conditions, delivering enhanced reliability and avoiding frequent failures of dryers under harsh operating environments.
Space Saving
The refrigerated dryer and desiccant dryer share one integrated base, reducing floor space. However, the compact skid structure leaves limited internal maintenance space, bringing difficulties to later servicing.
Convenient Installation
Piping for refrigerated dryer, desiccant dryer and filters is pre-assembled on one skid, reducing on-site installation work for customers.
Configuration Recommendation
Suggestion:
If site operating conditions permit, adopt independent refrigerated air dryer and micro-heat desiccant air dryer for combined operation. This layout facilitates independent operation and subsequent maintenance.
Disclaimer

This article is compiled and collected from online resources and widely reposted online. The authentic original author cannot be confirmed. 

If any copyright issue arises, please contact us, and we will handle it in accordance with relevant regulations as soon as possible.

The content reflects the viewpoints of the original author only, and does not represent the official position of this public account, nor does the public account take responsibility for the authenticity of the content.
Hyphenated Document Titles (for file naming)
two-common-problems-of-pressure-vessels-in-air-compressor-systems-does-your-facility-have-these-hidden-risks
real-data-revealing-the-untold-secrets-of-the-combined-dryer-industry