Majari Magazine

Control Scheme Improvement

by Rahadian Bayu on 29/04/08 at 12:07 am | 3 Comments | |

Exploration business is still facing challenges on its facility operations. There are many gaps of reliability improvement that will take big attention to be handled in order to reduce Loss Production opportunity (LPO) because of unreliable operation system. Casing Vapor Collection (CVC) Station is one of facility in exploration business that has function to condense vapors from casing line header which is separated inside CVC separator, after which the condensate is pumped to a production line heading to Crude Oil Gathering Stations. The sketch below is showing the produced fluid flow process in onshore oil field.

The transfer pumps are critical equipment on produced fluid flow process, because if the unplanned down is happening on transfer pumps, there will no fluid is able to be delivered to gathering station and requires the CVC station shut down. So that the availability of these pumps have to be monitored and take big attention on its daily performance in order to extend Mean Time Between Failure (MTBF).

The case study below of unreliable control scheme system has caused repetitive faliure on pumps and impacted to generate loss production from CVC station.

Once upon a time, there was a transfer pump repetitive faliure problem in surge vessel system. The current configuration of this system is shown below:

The current transfer pumps’ control scheme has generated a fluctuated level inside surge vessel and start/stop pump frequently in short time (Either Lead or Lag Pumps). This condition has led to pumps performance which set automatically based on liquid level status inside surge vessel (LI 09002). The lead pump sets “OFF” at 20% liquid level for safety device and will be “ON” at 40% liquid level. The lag pump sets “ON” at 60% liquid level and will be “OFF” at 40% liquid level.

Transfer pumps have experienced an unacceptable number of repetitive mechanical seal failures which often have resulted in oil spills. The investigation has been conducted in order to find out the bottom causes of this problem. One clearly identified cause is repeated start/stop pump frequently within 10 minutes has caused damage on mechanical seal. The frequent start/stop pump condition is caused by level system itself which is to set lead pump “OFF” at 20% and restart at 40%. This happened on lag pump as well, which requires lag pump automatically “ON” at 60% and “OFF” at 40% liquid level.

The level control loop does not work as well as the level control signal which is routed to recycle valve (LCV 09001) in maintaining liquid level but it almost no effect on level. The current system attempts to control level by interaction between the recycle valve control (“level”) and the product valve control (“pressure”). The interaction is too slow to be effective in controlling the vessel level, so the back up control (pump’s safety device) of auto-starting/stopping pumps is used.

The level setting point of liquid level inside surge vessel is previously 50% level and this was inducing liquid carry over in overhead (vapor) line heading to vent stack and misting was occuring due to unreliable control scheme on surge vessel system. The picture below showed an fluctuated level:

In order to eliminate this repetitive mechnical seal failures and reduce misting by providing reliable system which will control the level appropriately as dedicated. The current control scheme will be changed with control type as mentioned below:

1. Change the product valve control so that it is controlled by net product flow controller FIC-09001. This flow controller will have its set pont provided by a level controller LIC-09001. Level to flow cascade control is an industry standard and will provide good level control while still allowing constraints on the flow rate to ensure the pump(s) operate in safe range.
2. Change the recycle valve control so that it is controlled by the pump(s) discharge pressure controller PIC-09001. this will ensure that the pump(s) stay above the minimum flow dan provide good protection for the pumps.

This type of control system is typically used throughout upstream and downstream facilities as industry standard. The sketch below is proposed control scheme for replacing the previous one.

Here is the end result once implemented the proposed control scheme in surge vessel sytem and conducted a tuning process on its propotional gain (CP) and Integral (TR) values in order to set proper either pressure control valve and level control valve responses in operations.

By implementing the new control scheme at existing surge vessel facility, the frequent start/stop transfer pump(s) in short time is able to be eliminated. Since the fluctuated level has been decreased from +/- 45% becomes 5% only. By changing the level setting point from 50% to 40% has clearly identified that the misting process was able to reduced until 70%. These changes have been communicated thru updating the Standard Operating Procedure (SOP).