As with many things in life, it’s the unseen actions that often have the greatest impact in the end. Flushing of mechanical piping systems is often unseen, but can have an inordinate impact on the operation and longevity of mechanical systems, as well as commissioning of those systems. Commissioning, as defined by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) is “a quality-oriented process for achieving, verifying, and documenting that the performance of facilities, systems, and assemblies meets defined objectives and criteria.” While a future article will address the finer points of a well flushed piping system, what follows is an explanation of what flushing is and why it’s so important.
Flushing is the circulation of water at high velocity, sometimes hot, sometimes cold, often accompanied by chemical cleaning agents, for the purpose of breaking up sludge, oily film and debris, etc. These contaminants accumulate in piping systems during construction as the result of factory corrosion inhibitors, exposure to dust and dirt, slag and filings from welding and threading, and from joint sealant compounds. Once the adhered waste is loosened from piping interiors by high velocity water and chemical cleaner, and subsequently entrained in the water, the liquid waste is purged through a bleed and feed process, while solids are captured in mesh filters for disposal.
Inadequate flushing of any hydronic system will result in problems, both in the present and the future. Proper operation of heating hot water (HHW) and chilled water (CHW) systems are highly susceptible to inadequate flushing and will result in manifestation of various problems during startup and commissioning. HHW and CHW systems are loaded with numerous in-pipe devices, such as automatic flow control valves, flow metering devices and flow measuring instruments that can be easily plugged or disabled by sludge or debris. In addition, these piping systems contain fine mesh strainers at various locations that become easily plugged with contaminants. Unfortunately, these problems will often appear unrelated to flushing.
During the commissioning of these systems, a plugged strainer at a reheat coil can act every bit like a defective automatic control valve or a misadjusted circuit setter. Debris can lodge in automatic control valves and circuit setters resulting in what seems like a plugged strainer. A turbine type flow meter impaired with sludge can cause the instrument to report an inaccurate or erratic flow rate, thus running a pump at an unsuitable speed. A blocked port on a differential pressure transmitter or switch can signal a false reading. Oily film can coat heating and cooling coils resulting in reduced heat transfer efficiency. All these problems require time to troubleshoot, get to the cause, and ultimately fix. This directly impacts the efficiency of commissioning.
In a perfect world, flushing would be performed so that all traces of contaminate is removed from the piping system so that when a problem arises during commissioning it could be automatically assumed the issue is related directly to the device itself or perhaps system programming. In the real world, there is seldom a perfect flush.
The ease of troubleshooting and efficiency of commissioning is directly proportional to the adequacy of system flushing. Too often issues manifesting themselves as device problems are really due to inadequate flushing. So the next time you are in a commissioning meeting, don’t shy away from discussing flushing; not only is it proper etiquette, it’s a time and money saver.
– Timothy Schunk, Commissioning Team Member, HTS, Inc.