I remember my experience as trainer for chemical injection (CI) system on 2016. The attendees were operator, maintenance, and inspection/corrosion team. The course was focused on technical issue.
The course details related to various elemental aspects of the chemical injection system from design of the system including valves, pumping, monitoring, and delivery . It cover in depth of
- The various design aspects of the equipment involved.
- How to operate and monitor the performance of chemical injection packages.
We create pretest and post test to ensure the participant obtains benefit from the course.
Besides to transfer knowledge, the objective of this workshop and training is to increase level competence and reduce number of problems during normal operation.
Mostly on our site, chemical injection system is use to protect flow line pipes from corrosion. The chemical injection will maintain corrosion rate below allowable rate. It is injected on the X-mass tree upstream choke valve.
Chemical injection system will be tailor to meet the specific requirement. The schematic shown are typical schematic use to protect flow lines. So it might be different schematic CI for different application.T
Understanding the equipment involved play important role. If one equipment failure it will affect the entire performance. Even wrong setting on back pressure regulator (BPR) would disrupt the performance.
In our facilities, chemical injection system could be categorized of two types, as following.
- Multi injection system
- Single injection
The basic different of this system is how to set the flow rate.
In multi injection system flow rate are set individually on the flow control valve. This flow control valve is a pressure independent flow control valve. Once the valve is set at a desired flow rate, that flow rate is maintained even though the pressure conditions upstream and/or downstream of the valve may change considerably.
For single injection, flow rate is control using adjustment pump stroke length and stroke rate, without necessity to install flow control valve and back-pressure regulator. Pump should size properly in order to achieve target flow rate.
In addition, back pressure regulator and flow control valve is utilized for single injection with over capacity pump (based stock on hand) where injection flow rate is difficult to achieve using adjustment of pump stroke length and stroke rate.
Equipment of chemical injection system based on the schematic consist of
- Chemical injection pump
- Back pressure Regulator
- Pressure Safety Valves (PSV)
- Gauge drum
- Pressure Gauge
- Flow control Valve
Note: Back pressure Regulator and Flow Control Valve is using a well-known brand SkoFlo
Pump is reciprocating positive displacement pumps. The pump could be pneumatic driven or electrical driven. In multi injection system, pump will be operated based on highest well flowing pressure. Meanwhile stroke length and stroke rate is set at the total flow rate require for each well. Pump discharge pressure is recommended to set at: highest well pressure + 200 up to 400 psi + 13.8 barg
Note: +14 barg is really specific for flow control valve manufactured by SkoFlo valve
B. BACK PRESSURE REGULATOR (BPR)
It is specially design for multi injection; however it could be used for single injection if the pump is overcapacity.
Since on multi injection system, pump will deliver flow rate at total flow rate from each well, so overflow to the system network injection will occur. Back pressure regulator will dispense the overflow chemical injection back to the tank. I suggest the back pressure regulator is set at 80% of pressure safety valve.
Set point BPR is not regularly to be adjusted. Set point of BPR is also to maintain differential pressure between injection line and wellhead pressure at minimum 13.8 barg.
This is one of requirement of flow control valve which require minimum differential 14 barg between upstream and downstream to obtain stable injection flow rate.
C. FLOW CONTROL VALVE
It is design to regulate injection flow rate for each flow line. Unlike other control valve where flow will alter as system pressure changes, this specific flow control valve maintains a constant differential pressure across a fixed calibrated orifice, thus resulting in a constant flow through that orifice. As differential pressure is independent of system pressure changes, the flow rate is stable in all field conditions. However minimum 200 psi (13.8 bar) dP across the valve is required for the flow rate ranges.
D.PRESSURE SAFETY VALVE (PSV)
Pressure safety valve is not based on API 520. This safety valve has similar function as back pressure regulator, but it has different intention. BPR is intended for normal operation it will open and close regularly.
Meanwhile PSV will open for particular case, e.g.
- Blocked outlet by any isolation valve
- Clogged CI Injection line lead to over pressure
Therefore, PSV set pressure is at maximum allowable pressure / Wellhead shut in pressure.
During the training session, we heard many problems regarding chemical injection system from our operation e.g.
- Pumps not working,
- Plugging on the flow control valve,
- Seat inside flow control valve disintegrate
- Corrosion rate exceeded, etc.
My recommendations for these kinds of problems were
a. If pumps not working
Many causes could result pump not working. However, before examining the pump. I recommended checking pressure gas regulator + filtering regularly, since the supply is from hydrocarbon gas where water content might high. This would reduce the pump pressure discharge, after prolonged use without regularly drain the filter, this condition could lead the pump stop working.
b. Seat inside flow control valve damage / disintegrate.
This could occur if the flow control valve is located near the pump. Water hammering effect could occur during calibration where closing the valve handle adjustment rapidly. I recommended placing the flow control valve near the injection point. In addition it will optimize function of dampener.
I also find action by personnel which degrade the performance of multi injection system.
First, changing pump flow rate by lowering backpressure regulator set point. This action will affect the minimum differential pressure require by flow control valve if not properly calculated.
Second, bypassing one of the flow control valve due to plugging. This action will affect the entire injection flow rate to each flow line. By bypassing one flow control valve will disrupt the network pressure. The injection fluid will tend to route to the lowest pressure, in this case the line with bypass flow control valve. In consequence, the other injection line will not meet the required injection flow rate.
Third, Increasing orifice flow control valve intentionally. By increasing the orifice (Cv), it will change the original design of the flow control valve. The manufacturer has engineered and carefully selected the spring, chamber upstream and downstream, and the stem flow control valve. Increasing orifice will change the inherent Cv . In the end this action will disrupt the network pressure. Same consequence found on second cause.
Besides the problems, I receive valuable feedback from the trainee about the chemical injection system. Most of the concerns are not written on technical datasheet, catalog, even manual.
We summarize and make conclusion in order to maintain the performance and lifetime of the pump, as following
- If pump stroke is at the maximum 45 spm, the pump spring will often break below 1 month.
We decide to limit the stroke rate at maximum 40 spm.
- If stroke length shorten below half size of stroke length. It will often break the spring.
So we decide to maintain the stroke length at minimum 0.5
- Low speed of stroke/minute is more preferable than high stroke/minute in delivering the same flow rate
5 replies on “Chemical Injection System”
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