MEC Optimization 'ToR HDS-MEC'

Terms of Reference for an IMS-RCM study of an HDS unit

Introduction

After an initial RCM study was completed for the HDS (Hydro desulphurization) unit in the Westland Refinery, a series of small updates were applied over the years. As there was a quite a turnover of Reliability engineers, these modifications and updates led to inconsistency in the study. Although the business performance of the unit has been improved initially, management now expects that the TA interval of the unit to be firmed up to 4 years and to reduce total maintenance expenses (incl. TA) in line with Q1/Q2 of the benchmark.

It is further proposed to create a tighter connect between the activities specified by the RCM study and the annual preventive maintenance budgeting rounds, leading to an activity-based budgeting.

This Terms of Reference provides the plan for the MEC optimization of the HDS RCM study review, approved for execution by the Unified Petrol Reliability Manager, justifying the work to be done, the resources to be used and results to be reported out.

RCM and MEC optimization

RCM is a structured and multidisciplinary decision support process to effectively document the optimum maintenance requirements of physical assets in their operating context. The output of the RCM study is used for MEC optimization where the specified activities will be aligned later with other studying processes like PEI and SIS, to optimize Operational Availability and reduce cost via reduction of double work, improvement of effectiveness and efficiency.

For this MEC review the targets are:

• The RCM Analysis should bring the plant availability of the HDS unit from the quartile 4 performance to quartile 1/2 performance according last Solomon study.
• High priority repairs should reduce from 20% (Prio 1+2) to 5%.
• The MEC optimization should deliver a Turnaround interval of 4 years.
• Equipment with a criticality of M-level or higher are RCM analyzed in detail.
• Residual risk should be M-level or lower.
• An MEI score lower than 1 will be ‘run to failure’.
• The MEI score should preferably be larger than 2.
• The maintenance activities should be presented to enable efficient execution, where tasks that can be efficiently executed together are presented in groups.
• The MEC optimization is to be executed in the IMS web-tool when updates from the RCM, PEI and/or SIS studies are issued.

Plant data

The plant studied is HDS Unit, a catalytic chemical process to remove Sulphur (S) from diesel at 17500 Barrels per Day (2059t/d @ 740kg/m3). This plant has had some upgrades including new catalyst which is more reactive and requests a lower process temperature and pressure.

To reduce the Sulphur dioxide emissions from diesel in automotive vehicles, legal conditions demand a 50ppm Sulphur specification on the fuel, which has pushed the margin up to 300$/ton. The product loss equation for downtime of this unit is: 2059t/d x 300$/t = 617,710$/d or 25,740$/hr. There is no holdup or tankage in the System so any downtime results directly into production loss. In case of a plant-trip there is 4 hrs. extra downtime to recover production on specification.

Hydro-treating is a process to catalytically stabilize petroleum products by converting olefins to paraffin’s or remove objectionable elements from products or feedstock by reacting them with hydrogen.

The plant upgrades also led to new burners and furnace control System to reduce emissions and increase production. The upgraded burner control System is setup as a separate RCM functional System in IMS, including safeguarding Systems with instrumented protective functions.

Value levers

Current operational availability of the unit is 92.7% and 92.5% over last two turnarounds, which is in the quartile 4 group of the benchmark shown by Solomon studies.

Improvement is aimed for to reach 97% operational availability which should give a benefit of 6M$/yr. These improvements need to come from improved maintenance strategies on the HDS reactor, the recycle gas compressor, the furnace and heat exchangers, and the pumps in the unit.

The unit is subdivided into 4 functional Systems: Reaction section, Stripper section, Gas recycle section and Burner control System.

Function of the Reaction section is to treat 85t/h diesel and remove 99.9% Sulphur.

Functional failures of the Reaction System causing production loss are:

• Less than 85t/h diesel throughput of the System
• Unable to heat the product to 350°C at 85t/h.
• Unable to remove 99.9% Sulphur
• Product leakages with high H2S content causing plot clear.

The function of the Stripper section is to treat 85t/h diesel and remove 99.9% Sulphur/H2S.

Functional failures of the Stripper System causing production loss are:

• Less than 85t/h diesel throughput of the System
• Unable to heat the feed of 15.4t/h to 170°C at reboiler
• Unable to keep stripper top temperature to 35°C
• Product leakages with high H2S content causing plot clear.

The function of the Recycle gas section is to feed H2 rich gas to the high-pressure feed stream.

Functional failures of the Recycle gas System causing production loss are:

• Compressor fails to deliver discharge pressure at rated gas-flow (2400Nm3/hr.)
• High H2S content causing plot clear around the compressor (and off-spec purge gas).
• Product leakages with high H2S content causing plot clear.

The function of the Burner Control Fired heater section is to warm 85 t/h diesel from 250°C to 420°C respecting production plan and 'License to operate'.

Functional failures of the Recycle gas System are:

• Unable to warm 85t/h diesel from 250°C to 420°C
• High soot, High CO-CO2
• Unable to reach a 4y TA interval

The main Equipment in the HDS unit relevant for the RCM study are distributed of these four Systems with a total 58 Equipment and 24 Flocs.

An IMS-RBI study for the HDS unit is on the year plan for next year with implementation completed on the following year. Some pressurized Equipment do have inspection schedules attached. No SIFpro study is done.

Current maintenance activities involved are:

Pumps: These are typically 1-out-of-2 redundancy set ups and checked by operations as they are involved in the operator rounds to report issues. Operations take care of the main and stand-by pump operation. Current performance is a MTBM of 3.1 years with many seal failures and bearing failures. Also often the A and B pump are reported defect close after each other. Pump failures contribute for 33% to the unplanned downtime.

Compressor: A single installed reciprocating compressor that is E-motor driven and is continuously running. It is checked by operations through the operator rounds to report issues. Current performance is a MTBM of 1.3 years with many valve and piston failures. The compressor failures contribute for 35% to the unplanned downtime.

Heat Exchangers: These are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Current performance is that operations are unhappy with the heat exchangers and that there are often internal leakages and fouling involved. Inspection does not report fouling and requires cleaning for inspection. The heat exchanger failures contribute for 10% to the downtime.

Fired heater: The heater suffering from many trips. The furnace is inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Current performance is that end of run (close to next TA) operations has difficulty to heat the unit sufficiently. Inspection does not report fouling and requires cleaning for inspection. The fired heater failures contribute for 13% to the downtime and drives the TA interval.

Reactor: The reactor is inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. The chemical performance of the reactor is monitored every month, based on the lab analyses. The catalyst is renewed after 3-4 TA’s. In the last run of the catalyst the pressure drop over the reactor reaches the maximum pressure drop value leading to 10-15% reduced throughput, contributing to 5% of the production loss.

Pressure regulator: The control-valve is overhauled each TA although causing problems after each start-up. After 3-4 month in the TA-run the valve functions well, but has caused 5-10% lost production by then, contributing to 5% of the unit’s production loss.

Columns: The columns are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Every other TA there has been an issue with one of the columns, basically by poor maintenance from contractor workers causing internals to malfunction.

Vessels: The vessels – which are actually separators - are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Each TA there has been an issue with one of the vessels, basically caused by shortcomings with the internals.

HDS Downtime

The downtime of the HDS plant is given in operational availability 92.7% and 92.5% over last two Turnarounds (TA’s). Current TA’s have a 2 year cycle with almost 4 weeks downtime.
The main planned activities in the TA are inspections of the reactor and the heat-exchangers (with associated repair/ restoration work) ; and overhaul of the recycle-gas compressor.
The main unplanned downtime come forward from many pump breakdowns and compressor trips.
With this downtime the HDS is in the bottom quartile of the benchmark, where the ambition is to reach quartile 2 of the benchmark with an operational availability of 97%.

This table shows the current downtime and available of the HDS plant.

The ambition is to mover to a 4 years TA cycle and a 97% operational availability, with slightly reduced TA duration, and a reduction of on-planned downtime 13 days/year to 5 days/year by improved reliability of the pumps.

This table shows the desired downtime of the HDS plant after improvements made. 

Currently there is approval to repair all pumps on high priority, despite this approach the availability of the pumps in the unit is poor leading to frequent (short) downtime.

Asset Register HDS unit (Unit-X)

RCM Methodology References

For further background and details on RCM Analysis these key documents are referred to:

1. RRM S-RCM Manual, Shell Global Sol. V2 OG 04-30260
2. RCM II book by John Moubray; ISBN 9780831131463
3. SAE JA1011, Evaluation criteria for Reliability Centered Maintenance (RCM) Process
4. SAE JA1012, A guide to the Reliability Centered Maintenance (RCM) Standard
5. ISO 60300-3-11, Application guide Reliability centered maintenance
6. ISO 14224; Collection and Exchange of reliability and maintenance data for Equipment
7. IEC 60812; Analysis techniques for System reliability – Procedure for Failure Mode and effects Analysis (FMEA)