A few years ago, a major U.S. liquid pipeline company revamped one of its existing refined products pipelines. The pipeline provided a main supply of gasoline, jet and diesel fuel to many growing cities along the route as well as to two military bases. The project was designed to increase both throughput and reliability.
The existing control valves used for the line were ball-type, reduced-port, with old-style electro-hydraulic actuators that had become obsolete. These actuators were difficult to maintain because of parts availability and were prone to external hydraulic leaks. If the hydraulic fluid reached the ground, such leaks became recordable incidents in the field. The decision was made early in the revamping project to purchase new control valves and actuators for the entire project. Newer types of throttling electric actuators fit the requirements of the project. Here’s why:
The required applications for the control valves included back pressure control for the pumps, station inlet pressure control (required because of large elevation changes between some stations) and tank delivery pressure control. These applications were determined to have fail-last as the desired fail action for the actuators (a loss of 4-20 mA DC control signal or loss of power). Stroke time requirements were 3-5 seconds per inch of the nominal bore size of the control valve. The control valves selected had a required run torque range of 52,000 inch-pounds (in-lbs) (5,875 Nm) for the smaller ball valves to over 115,000 in-lbs (12,993 Nm) for the larger severe service ball valves.
The project team considered two different types of actuation technology: modern electro-hydraulic actuators and throttling electric actuators. Because they had been used in the past, the project team initially leaned toward electro-hydraulic actuators. However, based on the number of actuators required and resulting cost savings, the throttling electric actuators were chosen and ordered from the valve OEM.
INSTALLATION AND STARTUP
The initial installation and set-up of the throttling electric actuators in the field proceeded smoothly partly because the field technicians were already familiar with the technology—multiple motor-operated valves (MOVs) were already in place up and down the same pipeline. Hook-ups between standard MOVs and throttling electric actuators are the same except for the 4-20 mA DC control (as opposed to a discrete on/off control signal).
Everything operated smoothly until the pipeline’s startup. A couple of weeks after that startup, the motor thermals began to trip out on half of the actuators. Since this was not a desired result, an investigation began.
The actuators tripping motor thermals had setpoint changes more than 3,600 times in an hour (twice the actuator rating). This was due to several factors including:
- The proportional-integral-derivative (PID) control scheme
- The pipeline system response time
- The special nature of the severe service control valve in use
The initial setup of the control system was to monitor and change setpoint when required every 20 milliseconds. A throttling electric actuator could not keep up with the setpoint changes, which meant the actuator was continually hunting for its setpoint, and thus overheating.
Since the distance between pump stations was more than 100 miles long, the pipeline needed time to stabilize from process changes. This reality had to be considered for the control scheme. The customer collected data points and determined the process on different pipeline runs took from 4-7 seconds for the pipeline to stabilize from a setpoint change (referred to in Figure 1 as DT). Once the control valve was given a setpoint change, the pipeline would react like a degrading sinusoidal wave with an amplitude of 3-6 psig on small step changes.
The special hyperbolic nature of the severe service control valve used also presented PID challenges.
Figure 2 compares a hyperbolic Cv curve to an equal percentage (EQ%) curve and linear curve.
Taking the newly discovered information into account, it was determined changes were needed in the control scheme. The first change involved taking 100 raw pressure scans and averaging them out over the determined DT time period. This new time-weighted and averaged pressure reading was used to determine if the pipeline was operating within its desired pressure setpoint.
A second change placed software travel limits on the hyperbolic control valves to maintain a range in which movement of the valve actually made a difference on the process pressure (Figure 3).
One last challenge to the control scheme was that the station outlets were run at maximum allowable working pressure (MAWP). Every 14.7 psi of pressure consumption equaled an 8% cost to the pipeline.
To maintain the pipeline’s design parameters, a decision was made that when the station’s outlet hit MAWP, the control system would bump the control valve setpoint down 2 psig below MAWP (Figure 4).
Changes to the PID control scheme meant the pipeline operator could run the pipeline within 0.5% accuracy on pressure control, which was far better than the old-style electro-hydraulic actuators. The process went from 3,600 setpoint changes per hour to 30, which eliminated overheating of the actuators. Meanwhile, maintenance of the actuators to date has been much more cost effective, almost disappearing.
Given the cost savings and performance achieved in this first project, throttling electric actuators have become a new standard for the pipeline company involved in this case study.
Electric throttling actuators are a part of many successful process control environments. When used properly, and when fail-last on loss of power supply is acceptable, throttling electric actuators can offer a significant cost savings for new projects.
The following are the authors’ recommendation for liquid pipeline control valve throttling electric actuators:
- No actuator should stroke faster than 15 seconds. A good rule of thumb would be from 3-6 seconds per nominal valve size (inches).
- Make sure the conditions under which control valve run torques are based upon throttling are known; these can easily exceed valve break torques. Actuator sizing must take this into account.
- Actuators should be selected with a generous safety factor when used on throttling ball valves. As throttling ball valves wear over time, especially in non-lubricating refined product lines, the valve torque will increase. Actuator sizing should take this into account.
- Make sure the reaction time of the pipeline is considered for any PID control scheme.