4 replies to this topic

[Axis and Matrix]

Good day.

Our atmospheric pipestill crude charge pump motors have been experiencing increase in amperage reading, sometimes even exceeding the full load current which limits the feed rate of the unit. This first incident happened eight years ago, but the occurrences of the incidents have become more frequent. Please see attached simplified flow diagram below of the crude charge pump system:

Here are the following areas that our team is studying to intensify our understanding with the problem:

• Four instances of this high amperage incident were associated with the deposition on the pump impeller. We collected the deposits and had them tested. The results show that most compounds are sodium salts (carbonate, natroxalate, peroxide), with calcium carbonates and corrosion materials. With this, we implemented to dilute our caustic injection from 25 °Be to 8-10 °Be to reduce the amount of unreacted concentrated caustic to the system thus, reducing the chances of salt formation. Nevertheless, even with its implementation, deposition on the impeller continued to occur. Several studies reveal that caustic injection on the crude preheat train may cause fouling in the crude preheat. Do you have any experience of deposition on pump impeller or at least fouling of crude preheat due to caustic injection or any other sources? Is the location of the caustic injection as shown acceptable? What is the ideal caustic concentration to be used for this service? After impeller pull out, cleaning, and installation, the motor driver amperage readings were back to normal.
• There are also several instances when the motor amperage reading suddenly increases with the same crude diet (no change in pumping fluid). There are three pumps, each with a rated capacity of 30 MBSD. At this rated capacity, the pump operates at 61% of Best Efficiency Point (BEP), which is not on the recommended range of pump operation that is 70%-120% of BEP to avoid quick deterioration of pump parts. Since this is the rated capacity, can we operate at that flow rate without experiencing fast pump deterioration? The real problem is, we are operating 2 pumps in parallel at 35 MBSD, which means that each pump operates at 35.5% BEP. We have recorded incidents of bearing failure, severe vibrations and metal contamination of lube oil, which can be associated from this. Do you have experiences of pumps operating below 70% BEP (or in this case at rated capacity) causing high amperage reading to pump motor? Currently, we are using 1 pump with feed rate of 30 MBSD.

Our atmospheric pipestill is designed at 55 MBSD and with the test run of the pump, when feed rate reaches 35 MBSD, a single pump can deliver the desired flow rate, however, the amperage reading is nearing the full load current (35 A versus 36 A FLC) This means that after reaching a flow rate 70% of BEP, motor amperage reading is already limiting. Is our motor size insufficient for the pump service? Or is it okay since it we can operate the pump at 30 MBSD rated capacity (but at less than 70% BEP) with motor amperage reading not limiting?

• Due to ageing of the pump, it is given that there are internal roughness and wear on the pump casing which can decrease pump efficiency. This is due to the increase in friction between the fluid and the pump casing that need more energy to be able to pump the fluid at the same pressure/rate. We are exploring the use of rebuilding and efficiency coating on the pump casing/internal (example is Belzona coating) to address this concern. Do you have an experience on this and will you recommend other coating you’re currently using? By how much can the efficiency of the pump increase?
• Another factor is the crude feed being pumped. Since we started processing Russian crudes such as ESPO and Sakhalin Vityaz in late 2010, the frequency of occurrences has tripled. Although the team cannot completely accuse these crudes to be the primary cause (because there are several instances when even processing Russian crudes, the motor amperage reading is okay), you may have an experience on this.

If there are more causes that we fail to look at, kindly inform us anytime. Thank you for your time and we are hoping for your response.

Attached Files

•  Crude Charge Pump Process Flow.png   17.14KB   24 downloads

[S.AHMAD]

1. What is the salt content and BS&W of the crude? Have you provide enough time for settling and draining?
2. Is here any specific reason for the caustic injected upstream of E-1 & E-2? To my knowledge, the purpose of caustic is to neutralize HCl which is formed from hydrolysis of chlorides e.g Mg(Cl)2. It can be injected downstream of pump.
3. Has the crude tank being cleaned?
4. Based on the information available, the most probable cause of the charge pump desposition is from the crude tank coupled with higher temperature (since the pump is located downstream of E-2).
5. Some salts solublity decreases with temperature. This may explain why booster pump does not experience the same problem.
6. Relocate caustic injection point to downstream of charge pump may help but if caustic is the cause, then you are transfering the ptoblem from pump to heat exchangers.
5. Relocating the pump to upstream of E-1 is a possible solution but you may have pressure design limitation.
6. Probably, the crude tank requires cleaning. Sludge may have accumulated at the bottom.

[Axis and Matrix]

Thanks for the reply. Kindly see my response in red.

1. What is the salt content and BS&W of the crude? Have you provide enough time for settling and draining? Crude being fed has a 0.1 VOL% BS&W. We don’t have a regular monitoring of the salt content of our crude, but usual is <5 pptb. Crude settling must commence for 24 hours, however, there are several instances when settling happened for 4 hours for cases that crude tank was badly needed. Our Operations group makes sure that draining procedure is followed prior feeding.

2. Is there any specific reason for the caustic injected upstream of E-1 & E-2? To my knowledge, the purpose of caustic is to neutralize HCl which is formed from hydrolysis of chlorides e.g Mg(Cl)2. It can be injected downstream of pump. Yes, caustic is added to the system but to convert MgCl2 and CaCl2 to NaCl. This is done because NaCl is more stable (cannot be easily hydrolyzed) even under furnace temperature, unlike CaCl2 and MgCl2 which may hydrolyze at approximately 120 °C and 205 °C, respectively. We are also looking at the option of installing it downstream of pump due to its lower temperature compared to reaction (pumping temperature is at 96 °C) however, this might bring other problems such caustic induced cracking in the preheat exchangers.

3. Has the crude tank being cleaned? Yes. All crude tanks are not beyond their 10-year turnaround schedule. Sludge profiling may accurately survey the sludge content of the crude tank but the technology is expensive. This may be done on a later date.

4. Based on the information available, the most probable cause of the charge pump desposition is from the crude tank coupled with higher temperature (since the pump is located downstream of E-2). Can you explain this further? How can high temperature affect stability of deposition?

5. Relocating the pump to upstream of E-1 is a possible solution but you may have pressure design limitation. Yes, this may not be considered an option.

[S.AHMAD]

1. NaCl will be hydrolyzed at temperature greater than 500C which is not within the temperature range of CDU.
2. You have done good job at the crude tank side. Salt content is low enough to cause any trouble.
3. Relocating the caustic injection to downstream of pump will eventually confirm whether caustic is the root cause of the problem. Caustic is normally injected downstream of desalter. I presume you do not have desalter unit.
4. If the problem persists after the relocation, then the next suspect will be from crude and coupled with temperature. At this moment, I suspect temperature and centrifugal action of the pump contributes to the deposition of solid on the impeller.
5. Wait and see the result of caustic relocation.
6. Most salts, solubility (not stability) increases with temperature. However, some salts solubility decreases with temperature. For this type of salts, high temperature promotes deposition.

Edited by S.AHMAD, 05 February 2012 - 10:18 PM.

[Axis and Matrix]

Dear S. Ahmad, thanks for this.

We will consider recommending to the management to relocate caustic injection. Based on the deposits found, which is mainly sodium salts, fouling material maybe from unreacted caustic. We don't have a desalter unit. Do we expect this large amount of sodium salts coming from crude tanks?