Technical sharing: Overhauling the Underwater Pelletizing Extruder

Underwater Pelletizing Equipment (a.k.a. extruder) is one of the most critical types of equipment for petrochemical plants / the downstream production plants of the oil refinery industry.

How critical is it?

The most ideal case is to have zero equipment unplanned downtime and keep the machine running 7/24/365? to maximize production.

However, the technical information is not easily accessible from open sources such as the internet and books, and it is hard for mechanical engineers or fresh graduates to improve themselves, gain a basic understanding, or join the petrochemical industry without this information.

I am sharing the insights that I gain from the most recent extruder overhaul with the objective to encourage more information sharing in an effort to resolve the aforementioned issues.

Why is this article important to you?

1. You can understand more about the role and technical capabilities that one mechanical rotating engineer (petrochemical plant) should have.
2. Enhance your knowledge either for your personal interest or for career path development, if you would like to join the petrochemical industry as a mechanical rotating engineer.
3. As a platform for discussion. If you have any similar experiences, feel free to drop a comment below, or share this article.

Overview

1.The main objective of the overhaul

2.Preparation

• Manpower
• Tools
• Schedule and cost
• Prepare for the unexpected findings

3.Selected technical highlights

• Gear pump rotor installation
• Special timing gear coupling installation

I can’t share too many details such as the maker, model, year of manufacture, drawing and etc. about the machine as I am not sure whether the information is private and confidential. However, I can share some basic info.?

The equipment maker is from Japan, and the machine was made almost 30 years ago (yes, it is older than me). Even the existing team of specialists from the maker doesn’t have complete experience in overhauling this type of equipment. Thus, it would be something very valuable for you, and your company, if they use this kind of equipment and need someone with such experience.

1. The main objective of the overhaul

The main objective of the overhaul is to prevent unexpected failure of the equipment, and for such critical equipment, we are still practicing the concept of time-based maintenance. We do condition-based monitoring and we gain good insights from it as well, but condition-based maintenance is not practical in this case as we can’t stop the machine anytime we want.

In general, the equipment maker recommends replacing the consumables such as the seal and gasket every 2 years, replacing the bearings, inspecting the major parts such as the screws and gear shaft every 5 years, and replacing the screws and gear shafts every 10 years.

However, we should take note that it is reasonable for equipment makers to give recommendations based on both commercial reasons and the average lifespan of the equipment. So sometimes you would find the maintenance interval is too short.

If the machine is well operated, the installation and maintenance are done well, and the correct parts are installed, then the oil seal and ABC seal can last for 3 - 4 years, and above. The bearings can last for almost 10 years, and the major parts can last for more than 25 - 30 years.

However, some frequently moving parts such as the UWC (Underwater Cutter) might need a more frequent overhaul or a mechanical seal replacement.

All the above statements are justified by the maintenance records.

2. Preparation

Before we talk about the preparation, let me ask a question.

What are the KPIs for an overhaul?

There is no absolute right or wrong, but my answer would be: Schedule, cost, workmanship, quantity of unplanned incidents, and the comprehensiveness of the inspection record.

In the petrochemical industry (as well as the oil and gas industry), time is the essence of revenue. Equipment downtime will reduce production and thus the profit of the company.

Regarding maintenance cost control, it is good to minimize the change of spare parts and cost of manpower, however, it is more important to ensure the reliability of the equipment since most of the time we are expecting the equipment to operate under high temperatures and pressure for more than 5 years.

Schedule, cost, and workmanship are very general KPIs for almost all industries. So, what I want to highlight is

• KPI #4 quantity of unplanned incidents,
• KPI #5, the comprehensiveness of the inspection record.

Unplanned incidents could be an unexpected scratch on the shaft, unexpected runout of the shaft during the inspection, etc. The unplanned incidents will greatly affect the cost and schedule control. What's even more worrying is that more unplanned incidents reflect that the team who takes care of the equipment might not have sufficient understanding of the equipment, or the equipment has some unseen changes due to aging. All these subsurface risks shall be well-recorded for evaluation and monitoring.

Thus, it brings us to KPI #5, the comprehensiveness of the inspection record. Without the proper record, it will be really difficult for us to evaluate and monitor the equipment's condition. Well, the machine doesn’t speak using our languages, but it does tell you a lot of information during the overhaul. So I think another crucial point that both management and engineers shall take note of is, if we only focus on disassembly and reassembly, skipping all the inspections is similar to putting the team at risk because we won’t have sufficient understanding of the machine's condition for evaluation, monitoring, and root cause analysis (RCA).

In my humble opinion, there are 4 categories of preparation work:-

1. Manpower
2. Tools
3. Schedule and cost
4. Prepare for the unexpected findings

To keep this article short (I hope it is), I won’t go into details.

Manpower skillsets and arrangements

• Rigging, alignment (laser and dial gauge), hydraulic and manual torquing, taking measurements, millwrights’ basic skills, and understanding of the work procedures (a pre-work briefing is very important).
• It is recommended to have 2 shifts team, where the day shift will work on disassembly, inspection, and assembly. The night shift shall focus on cleaning activities and other non-critical jobs.
• It is good to assign a full-time site supervisor and a full-time workshop supervisor to work at the same time to keep the schedule on track without sacrificing the work quality.

Tools

• Precision measuring tools, special tools (for mixer screw removal, UWC overhaul, screen changer removal), lifting tools, big torques, hammers, consumables (Threebond, blue contact, fine oil stones, Scotchbrite, sandpaper, cleansing agent, etc.). I keep a list of tools for my own reference.

Schedule & cost

• The equipment maker shall be able to provide you with a common and general schedule. However, considering the overhaul history, tools completeness, skills of the team, and many factors, it is always recommended to increase the number of days by either 1 week, or 50 - 70%, and increase the number of shifts to 2 shifts.
• The equipment maker schedules the overhaul based on the Japanese working culture and their knowledge and skills, so it is reasonable that their schedule is always shorter than ours.
• I am not allowed to disclose any information about the cost.
• In general, the waiting time for getting work permits, lunch and dinner rest hours of workers, and the lack of knowledge for work coordination and synchronization have greatly consumed 30 - 40% of the time. This is something that can be resolved immediately so it is something that the management should pay attention to.

The unexpected findings

• Sometimes, it would be difficult to dismantle the coupling hub, bolts, and nuts. Sometimes a forced removal will cause the shaft to be damaged, so you need to have urgent repair.
• In addition, the gearbox bearing cover needs machining work, or you need to prepare one complete piece of metal shims to control the axial movement of the bearing.

3. Selected technical highlights

General description of the extruder assembly

I assume that some of the readers are not familiar with the extruder, so I will briefly mention the individual equipment of the extruder system. In general, the extruder system consists of a mixer motor, mixer gear reducer, starting unit gearbox and motor, mixer screws, cylinder (housing of the mixer screws), slot, diverter valve, gear pump, screen changer, screen changer, die holder, die plate, underwater cutter bowl and cutter unit, cutter unit motor, timing gear, and special coupling (only for old design), universal joint, gear pump gear reducer, gear pump motor.

I select two critical jobs

1. Timing gear special coupling installation
2. Gear pump rotor installation

In the engineering industry, we practice using ITP (inspection test plan) to identify the inspection point and hold point (hold point is a point where the specialist or the end user must witness and approve before the workers can proceed to the next step).

In overhauling critical rotating equipment like extruders, I would say almost all points are hold points, except cleaning and general coupling alignment work, because most of the rotating workers can understand and perform general coupling alignment work.

3.1 Timing gear special coupling installation

• It is important to measure the shaft's outer diameter and the coupling hub's inner diameter prior to installation. There should be some clearances between the shaft and the coupling hub, however, it is a good practice to verify it and then heat the coupling hub before installation.
• If there is an O-ring sitting inside the coupling hub, the heating temperature shall not exceed the O-ring maximum allowable temperature, however, the rule of thumb is to heat and expand the coupling hub by a certain amount prior to installation.
• Once the coupling hub is installed, immediately check the gap between the hub and the nearest step (stopper) to ensure that there is no gap. Fix the coupling hub in position and cool it using air blow.

• The gear coupling comes with certain markings, thus we should always follow the marking done by the manufacturer rather than the old marking during the disassembly. If the old marking deviates from the manufacturer's marking, we shall follow the old marking because the gear teeth could have worn out throughout the operation.
• The special gear coupling is made to take more than 2mm of misalignment, however, it is a good practice to perform alignment to below 0.1mm. Due to the big backlash of the special gear coupling, the usual laser alignment tool is not preferred. We use a water level gauge (0.02mm), dial gauge, special tool, and inside micrometer to perform the alignment work.
• Later, we need to set the backlash of the gear pump rotor, and then machine the key of the special gear coupling to lock it in position. I will provide some photos and descriptions in the next paragraphs of this article.

3.2 Gear pump rotor installation

• The gear pump rotor will have scratches or damage if the backlash is not set correctly.
• The cleanliness of the gear pump rotor and casing, the physical dimension (evenness) of the rotor casing, and the bearing are crucial. We need to measure the evenness and the inner diameter of the gear pump rotor casing, and the flatness of the side cover.
• It is recommended to use an overhead crane to lift and install the gear pump bearing, rotor, and casing. First, we install the NDE bottom bearing, followed by the top bearing, then the side cover. Next, we install the bottom rotor and bottom bearing, followed by the top rotor and the top bearing, and the side cover. Finally, we install NDE and DE gland seals.
• It is crucial to measure the flatness of the bearing after each installation. After that, we shall torque the side cover and check the axial movement of the rotor.
• While setting the position of the gear pump rotor, we should lock the bottom rotor and the drive parts of the rotor. The only parts that can rotate are the top rotor and its coupling. We shall take at least 5 measurements to confirm the total backlash, then shall machine the key to ensure the rotor will be fixed in this position until the next overhaul.
• It is acceptable to use a feeler gauge to take the reading when we want to know how much to machine off from the original key. The keyway is tapered i.e. the bottom is smaller and the top is bigger, thus, you can’t do one-time machining to fit the key into the keyway. In fact, you need to file and remove the additional material from the key.
• Usually, several hours of trial and error are needed to get the correct dimension. Kindly ensure no gap between the key and keyway, otherwise, the rotor position will change during the operation.

In Malaysia, the number of such extruders is very little, compared to the number of pumps, compressors, blowers, etc. common equipment. We do have a lot of compounding extruders but the size and criticality, the technical difficulties are different from the big capacity extruders.

I hope this article has given you a general view of the extruder overhaul in terms of project management and the technical knowledge required for workmanship control. I tried to optimize both the content and the length of the articles, and I hope it is not too long for you.

Share this with your friends, or tag them in the comment session, if you find this article useful for them. If you have any comments that you would like to share with me, feel free to drop me a comment or message.

Engineering is about finding the most feasible solution(s) both technically and commercially.

Thanks for reading!