Valve actuators are a common necessity along vast stretches of pipeline infrastructure. Out in the middle of nowhere, line power can be less than dependable, costly to install on site, or is simply not available. Solar powered systems and line powered UPS systems can provide both the power and reliability to remotely operate a valve when the need arises.
In reviewing energy requirements first, a typical remote valve actuator site will have some sort of communication, whether it’s licensed radio, cellular, or satellite. This same site will likely have a small PLC or SCADA system. In the event of a power failure, several days of UPS backup power is desirable – enough time to get a person in a truck to the site.
In the case of a solar powered system, five days or more of backup time (autonomy) is required for proper system function during bad weather. Assuming a 40-Watt load for PLC, SCADA and communications, operating at 24 Volts DC, a system requires 200 Amp-hours of battery capacity. This value increases to 313 Amp-hours when the battery is properly de-rated for Depth-of-Discharge and End-of-Life. The energy requirements will vary, but even the smallest systems will likely have 100 Amp-hours of battery capacity.
The instantaneous power required by a valve actuator depends on the application. Electric motors from less than 1 horsepower to several 10’s of horsepower are common. While the power requirements vary wildly, what is common across the spectrum is the relatively small amount of energy required to operate a valve. By comparison, a 10 horsepower actuator operating over a 2-minute cycle requires about 333 Watt-hours of energy. Converting this value to Amp-hours in a 24 Volt DC battery, about 14 Amp-hours are required to close a large valve in a hurry. At Solarcraft, we consider this a “relatively small amount of energy” because typically batteries used for remote power are 100 Amp-hours or larger.
The primary load in remote systems is nearly always the electronic equipment – not the actuator. Often, a small solar array or for that matter 120 Volt single-phase line power (brown and dirty) is all that is needed to satisfy the energy requirements of a system. A small system can indeed power a large actuator.
The power requirements of a remote actuator system are an entirely different matter; especially where standard AC motors drive the actuator. AC motors, whether single phase or 3- phase, require an inverter that can supply the majority of the published locked-rotor current of the motor. Locked-rotor current can simply be considered the “starting” current of the motor, usually somewhere between 4 to 10 times the “running” current. Because actuators may sit for extended periods between operating cycles, static friction in the mechanism becomes a factor in starting. Ensuring that sufficient current is available to start the locked rotor and overcome the friction is vital.
Supplying the entire locked-rotor current rating of the actuator motor is not necessary. Experience has taught that the maximum current capability of the inverter should be 6 times the running current of the motor. Inverters installed in Solarcraft systems are capable of delivering 2 times their rated output for 3 seconds; this is sufficient to overcome the demand for starting current. Therefore, a good rule-of-thumb is to provide 3-kiloWatts of inverter power for each horsepower of actuator motor. For example, an 8 horsepower motor requires a 24-kiloWatt inverter. The inverters are modular and can be configured for power output, single, split and 3-phase operation. However, these inverters are a major component of the overall system cost.
Depending on the size of the valve and the stroke time required, there are alternatives to consider. Several manufacturers of actuators offer 24/48 Volt DC powered versions. These eliminate the need for an inverter altogether because system batteries are more than capable of supplying starting currents. A 24/48 Volt DC actuator offers substantial cost advantages and should be considered whenever a smaller actuator is required. 110 Volt DC versions are also available but should be avoided for solar powered systems. Switchgear, fusing and product availability become vexing issues when designing systems beyond 48 Volts DC.
Depending on the actuator size (2-3 horsepower or less), a single-phase motor can be chosen over a 3-phase. While the single-phase motor is somewhat more costly, the invertor to power it is less complicated. This more than offsets the increase in motor cost.
The voltage of the actuator motor is also an issue. The industry habit is to specify 480 Volt 3-phase motors. In the case of Solarcraft’s inverter, 120/240 single/split phase and 208 3-phase are the only options for output voltage. Voltages can be stepped up with transformers if needed. This is often done where an existing actuator is being upgraded.
In the case of new installations, 208 Volt motors simplify the system; and all but the very largest actuators are available in 208 Volt models.
To recap powering remote valve actuators, remember the following: