[From：http://instrumentation.web.id] [Author：admin] [Date：11-09-13] [Hit：]
In the wellhead control panel there is a Hydraulic Power Unit (HPU) that utilize hydraulic reservoir, hydraulic pumps, and accumulator as it components. The hydraulic pumps is used to pressurized the hydraulic network (hydraulic line to valve actuator) and charge the accumulator. (See Introduction to Wellhead Control Panel). Since the hydraulic pump plays an important task, then we need to size it correctly so that the functional requirement of the HPU is achieved.
First of all the calculated capacity of the pump will depends on the actuator operating pressure, speed of response, system capacity and capable of providing successive well startups in the specific time required for the full complement of the platform well slots.
In the general rule of pump sizing, it is an obligation to have NPSH calculation with the acceptance criteria NPSHa greater than NPSHr by at least 2 feet. But for Hydraulic Power Unit on Wellhead Control Panel applications, with flooded suction (pump is located on the bottom outlet of the hydraulic reservoir), NPSHa will generally exceed NPSHr by 10 or more feet (of pumped fluid) – this is why NPSHa is normally not a consideration in wellhead control panel applications. This is reasonably since the flow rate of the pump is usually very slow compare with any other process pump. This slow flow rate usually also correspond to the pump NPSHr. Higher the flow rate, higher NPSHr is visible. Further more, the pumped hydraulic oil isn’t volatile and its vapor pressure is almost on vacuum pressure, this characteristic will make the NPSHa higher than pump used for water fluid. Therefore most of the pump sizing calculation done by the vendor isn’t including NPSH calculation in their report.
Before start to calculate and select the pump capacity, the following data is needed (see Hydraulic Accumulator Sizing for Wellhead Control Panel Hydraulic Power Unit (HPU) for further info regarding accumulator sizing):
- Required hydraulic volume to charge the accumulator and hydraulic network from zero to its minimum operating pressure. Vn
- Required charging time to charge the accumulator and hydraulic network from zero to its minimum operating pressure. Tn
- Required hydraulic volume to charge the accumulator and hydraulic network from its minimum operating pressure to its maximum operating pressure. Vn2
- Required charging time to charge the accumulator and hydraulic network from its minimum operating pressure to its maximum operating pressure. Tn2
After we got all above data, we can do the following simple sizing:
Pump Required Flow Rate Q = Vn/Tn
Pump Required Flow Rate 2 Q = Vn2/Tn2
Maximum system operating pressure = 3500 psig
Maximum system operating pressure = 3500 psig
From this calculation, we can look at the pump vendor catalogue and select the pump that has flow rate at least 20 Ltr/min and maximum output pressure at least 3500 psig. Let’s say from pump catalogue we select the following air driven pump:
Pump flow capacity = 22 Ltr/min
Maximum output pressure 4000 psig
From above selected pump data we can conclude the following result:
18 Ltr / (22 Ltr/min) = 0.818 minutes
100 Ltr / (22 Ltr/min) = 4.54 minutes
The pump size is properly selected since the charging time is achieved.