What is a Siphon and How Do You Account for One When Designing a Piping System?

By Jeff Sines, Product Engineer, ESI

When designing a piping system for any industrial or commercial facility, it is important to understand how and where a siphon may occur, and whether the siphon will be working for you or against you during system operation. Failure to take the siphon effect into account can lead to collapsed tanks and pipelines, downtime, lost production, safety hazards, and environmental excursions.

A siphon is a natural phenomenon that can occur in any piping system that transports a liquid from a higher elevation to a lower elevation. As gravity pulls the liquid in the piping downstream of the high point, the flow of liquid can create a vacuum at the high point that pulls on the liquid in the upstream piping, allowing the flow to continue. Anyone who has used a garden hose to siphon gas out of a car’s gas tank is familiar with this phenomenon of fluid flow.

As a Shift Superintendent at a pulp mill in the 1990s, I encountered several situations in which a siphon was not accounted for and resulted in damaged equipment, downtime, lost production, and environmental cleanup costs. One such situation that took everyone by surprise occurred when the mill was shutting down for a routine maintenance outage to repair minor deficiencies throughout the mill. One maintenance task involved welding a leak on the recirculation loop of the Low-Density Stock Tank in the machine room, shown in the PIPE-FLO® Professional model in Figure 1.


Figure 1. Consistency control of low-density stock in a pulp mill.


During normal operations, stock in the Low-Density Stock Tank was pumped by the Low-Density Stock Pump to the Pulp Machine through the Stock Flow Control Valve. Some of the flow was sent back to the Stock Tank through a Recirculation Loop. Fresh stock from the Bleach Plant was mixed with Dilution Water and recirculated stock to maintain the consistency of the stock at about 3%.

A small leak had developed at the weld of the pipe going from the Recirculation Loop to the Stock Flow Control Valve, located about 30 feet above the ground. It was more of a messy nuisance than an operational problem, so the leak wasn’t repaired until it grew big enough to get placed on the minor maintenance list. The machine had to be shut down to drain the pipe and weld the crack for repairs. The operator conducted a proper shutdown of the system, then aligned the system to drain the 12-inch Recirculation Loop, as shown in Figure 2, and then proceeded to lock out and tag out the system.


Figure 2. Recirculation Loop aligned for draining.


When the drain valve was opened on the 4-inch drain line, a steady stream of bright white bleached stock flowed at high velocity to the ground around the tank and pump, which was enclosed inside a berm. Since the Recirculation Loop was a big pipeline and would take awhile to drain, the operator continued down his task list to lock out other equipment for maintenance. He anticipated a little clean up when he returned to prepare the area for the welder.

After about 30 minutes, he returned to the drain valve expecting the flow to be stopped and the Recirculation Loop to be empty. But instead, the stock was steadily draining at the same flow rate, filling the berm to about ankle deep around the pump, and it’s motor. Looking at the 12-inch pipeline, the operator surmised the continued flow was due to the large volume of the Recirculation Loop and it just needed a little more time, so he continued with the lockout and tag out maintenance for the rest of the equipment.

He returned to the drain valve after about 45 minutes, having been delayed by other tasks on his list. Surely the Recirculation Loop would be empty by now, he just hoped he wouldn’t have to spend too much time hosing up the mess. As he approached the berm, a huge surprise awaited. At least a foot of stock surrounded the pump and motor with the level about to rise above the motor base plate. He could tell the stock was still flowing strongly from the drain line, so he waded into the mess and closed the drain valve. He then called me on the radio to let me know there was a problem.

As I surveyed the scene and listened to what had happened, I too, was confused as to where all this stock was coming from. The Recirculation Loop wasn’t THAT big. I traced the piping with the operator to verify the lineup was correct and found no discrepancies with how the system was aligned. Just then, another call came across the radio: the Machine Tender had been losing liquid in the Low-Density Stock Tank even though he had shut everything down over an hour ago.

Aha! We now knew where the stock was coming from, but how? The line from the tank to the pump was properly isolated and locked out, and the Recirculation Loop discharged to the top of the tank, or so we thought. A closer inspection of the stock tank was in order. After opening the top cover, we saw that the Recirculation Line was submerged and discharged below the liquid level in the tank. Taking a step back to see the big picture, the Low-Density Stock Tank had dumped about five tons of prime product on the ground. We had created a siphon!

Lesson learned; to clean up the stock in the berm this incident resulted in labor cost,the loss of about $10,000 in the prime product, and unplanned environmental cleanup costs.

Piping systems in industrial and commercial facilities can be extremely complex and difficult to understand, even for the most senior engineers, supervisors, and operators. A siphon exists anytime there is an elevation change from a higher elevation to a lower elevation. A system can be designed to take advantage of the siphon effect, but often the unintentional creation of a siphon will occur and engineers may not even be aware, this type of siphon will cause problems that are difficult to troubleshoot and resolve. These problems may damage equipment, create safety hazards, cause downtime and lost production, or incur additional environmental cleanup costs.