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Filter Blow-By & Follower Pointer

Filter Blow-By

Filter blow-by is the dumping of particulate, through a filter element, into the downstream system. This can be caused by overloading or clogging a filter to the point of failure. This failure can come in the form of tearing or perforations of the filter element or seals. This dangerous condition can foul the process fluid and cause thousands of dollars in equipment damage.

Imagine a location such as a remote gas pipeline station where there is often no one to monitor the instruments and equipment on a regular basis. This is becoming more common as budgets decrease and automation increases. Now one person is often responsible for maintaining several locations.

Filters are an essential component of those stations, ensuring clean gas and protecting the pumps, meters, regulators and other sensitive and expensive equipment. If that filter in that remote location fails there may be no one there to notice and correct the problem.

Filters are usually monitored by differential pressure gauges or transmitters. Differential pressure transmitters can send a signal that allows for remote monitoring but more can be done to assist the person troubleshooting the equipment problem.

In a remote location a DP transmitter or gauge cannot alone relay that there is an alarm condition. A person may arrive at the remote station and find the filter DP gauge readings are fine, with low differential pressure. What they may not know is there was recently a very high differential pressure condition that suddenly dropped when the filter element failed emptying the contents into the system.

Under those conditions the low differential pressure readings indicate healthy filter operation even while particulate is causing damage.

Follower Pointer – the Simple Solution

A simple, low-cost follower pointer can be added to the differential pressure gauge to allow the troubleshooter to identify the problem. A follower pointer is red in color while the DP range pointer is normally black. During set up the user sets the red follower pointer against the black DP range pointer and walks away. The follower pointer itself is an assembly that includes the lens and red pointer with a small knob to set the position of the red pointer.

During operation the black DP pointer pushes the follower pointer to the highest reading but the follower pointer does not follow it back down the scale if the differential pressure drops. It will always remain at the highest differential pressure reading until it is reset.

If filter blow-by occurs the red follower pointer would have been pushed to the highest differential pressure reading experienced since the last visit. Even if the black DP range pointer is showing normal range of operation the follower pointer can indicate an alarm condition by being in a much higher position than the black DP range pointer. This separation of the black and red pointer immediately signals the troubleshooter that closer inspection is required.

When the failure of the filter occurs the differential pressure gauge or transmitter reading drops back to a lower level since the backpressure caused by a loaded filter condition no longer exists while flow is unimpeded. Without the higher position of the red follower pointer the low reading of the differential pressure gauge would mask the problem.

Redundant Readout

Even if a differential pressure transmitter is already in place it is helpful to add a separate redundant DP gauge with a follower pointer as a local readout so identifying filter blow-by is identified and corrected immediately. The telltale aspect of the follower pointer is easy to understand and fast. Even if there is a data logger recording the DP transmitter reading history, the follower pointer is still very helpful.

When the filter situation is corrected simply reset the follower pointer against the black DP pointer so it is ready for the next time.