Recommended By-Pass Valves on Regenerative Desiccant Compressed Air Dryers

Pre- and after- filters are essential features on any type of regenerative desiccant dryer.  Filters remove compressor lubricant and desiccant dust from the air stream.  However filters often require service at an interval that differs from the service schedule of the dryer.  By-pass valves and piping are often used to isolate components in the air treatment system to allow for service without shutting down or blocking-in the entire air system. 

What is the best layout for by-pass piping and by-pass valves to accommodate the service requirements of both filters and dryers? 

The answer to this question depends on the particular circumstances in the application.  So let’s take a closer look at some of the issues in play, so that you have the tools to make an informed decision about the by-pass approach that best meets the needs of your compressed air system.

 Three-Valve By-Pass

The most basic configuration for by-pass valves and piping uses three valves, with the layout shown below.




This configuration allows an operator to isolate the compressed air treatment equipment as a whole.  When the by-pass valve is open and the block valves are closed, wet air will be delivered to the plant.  Is this acceptable for the time period the equipment is under service?  If not, you’d better have a redundant dryer in parallel to the by-passed unit.  Otherwise, the 3 valve by-pass configuration is not for you.


Nine Valve By-Pass

A more extensive valve and piping configuration is shown below.


This configuration is probably the most common as it allows for the isolation and service of individual air treatment components.  Although there remain some notable points of caution.

  • Isolating the pre-filter as shown above will potentially deliver oil laden compressed air to the dryer.  This risks damaging the dryers inlet valve or valves and also could contaminate the desiccant bed.  Both categories of damage are potentially costly.  I recommend this configuration only when operating an oil free compressor.
  • Isolating the after-filter as shown above will potentially send desiccant dust to the plant.  Granted, the amount of desiccant dust produced during the 5-10 minutes you’ll need to change a filter element is not great.  You’ll need to weigh the risk in proportion to the sensitivity of your air-dependent processes and pneumatic equipment. 
  • Also, many plants have point of use filtration upstream of critical equipment.  If this is you, then briefly by-passing an after filter should not be a cause for alarm.


11 Valve By Pass

Here is another configuration, with an additional layer of operational flexibility.


Here there are redundant filters piped in parallel.  This layout avoids the pitfalls that are inherent in the 9 valve configuration.  Back-up filters eliminate the risk of damage to the dryer and mean no desiccant dust will be sent to the plant.  The obvious weakness point of this configuration is that if the dryer itself needs service and is by-passed, wet air will be sent to the plant.  Fortunately the service internal on a regenerative compressed air dryer is much longer than for its filters.  Most plant operators schedule service on the dryer during planned shut-downs.  With the 11 valve configuration the more frequent filter element changes can be done on the fly.

If your operation runs 24 hours per day, 365 days per year, and requires instrument quality compressed air 100% of the time, then no by-pass configuration will meet your needs in full.  You’ll need a fully redundant, parallel compressed air treatment system.

TECH TIP: a by-pass valve that is unintentionally left open or cracked will contaminate your air system.  This is a leading source of wet compressed air in systems whose air treatment equipment appears to be working normally.

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