RCM Analysis 'Term of Reference HDS'

This tutorial provides an example ToR document, which is used in the IMS RCM online tutorials / manuals.  It holds relevant data for an RCM study. An RCM ToR document is recommended to be drafted when a RCM study is to be done. With management approval of the RCM ToR, a better plan is provided to execute the RCM study and to get approval for the end result, allowing it to be operationally implemented.

This ToR document enables us to illustrate how RCM information flows from the preparation phase to the actual registration in IMS-RCM, and allows the RCM workshops to be conducted with IMS-RCM.

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

Introduction

After the Solomon study done on the Westland Refinery of the Unified Petrol company last year, an RCM study was scheduled to be executed for this year. This ToR provides the plan for the RCM study of the HDS (Hydro desulphurization) unit in the Westland Refinery to be executed in IMS and approved for execution by the Unified Petrol Reliability Manager, justifying the work to be done, resources to be used and results to be reported out.

RCM

RCM is a structured and multidisciplinary decision support process to effectively document the optimum maintenance requirement of a physical asset in its operating context. The general objective of RCM is to improve reliability and availability of Equipment by selecting appropriate PM (Preventive Maintenance) tasks and techniques without jeopardizing Health, Safety and the Environment. A RCM review targets to develop a cost-optimum proactive maintenance plan that will meet the site’s targets in integrity, reliability and HSE. The RCM process used focuses on dominant Failure Modes which are assessed in terms of its risk to the business, indicated as a Criticality, and driving the maintenance effort to reduce the risks to tolerable levels. A cost-benefit Analysis is carried out by comparing the consequences of ‘doing no Preventive Maintenance’ and the cost of ‘doing recommended Preventive Maintenance’ to justify and optimize the PM tasks recommended. This optimization can involve modification of existing tasks, adding new or even delete maintenance tasks if it appears to be not cost-effective; and is expressed in a MEI (Maintenance Efficiency Index) indicating there is economic benefit in doing the maintenance.

A multidisciplinary team coordinated by RCM facilitators carries out the RCM study where the team originates from the plant’s maintenance and operations engineers and crafts – using site specific and Equipment specific data, documentation and local experience. To this end the RCM assessments stored in the IMS-RCM tool come forward from decisions agreed in the multidisciplinary team.

For this RCM study the targets are:

• The RCM Analysis should bring the pant performance of the HDS unit from the quartile 4 performance to quartile 2 performance according last Solomon study.
• High priority repairs should reduce from 20% (Prio 1+2) to 5%.
• The IMS-RCM Analysis should support a Turnaround interval of 4 years.
• Equipment with a criticality of M-level or higher are 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 amount of lower criticalities in the study (N and L) should not be greater than the amount of higher criticalities (M, MH, H and E) (so max 50%).
• The maintenance tasks recommended should be presented in a way to enable efficient execution, where tasks that can be efficiently executed together are presented as a group.
• The Analysis is to be executed in the IMS-RCM web-tool and reported out in a period of 4 months from sign-off date of this ToR.

Plant data

The plant studied is the Hydro desulphurization (HDS) Unit, a catalytic chemical process to remove sulphur (S) from diesel at 3500 Barrels per Day (412t/d @ 740kg/m3). To reduce the sulphur dioxide emissions that result from using diesel in automotive vehicles, tight product specification on sulphur are provided, demanding sulphur removal. Depending on the market position - meeting the sulphur specification increases the margin of the diesel-product by an average 150$/ton. The product loss equation for downtime of this unit will be: 412t/d x 150$/t = 61,771$/d or 2574$/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 4hrs 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 diesel feed is mixed with hydrogen-rich gas before it is preheated to the reactor inlet temperature of 350 °C. The oil feed combined with the hydrogen-rich gas enters the top of the fixed-bed reactor. In the presence of the metal-oxide catalyst, the hydrogen reacts with the oil to produce hydrogen sulphide (H2S), ammonia, saturated hydrocarbons, and free metals. The metals remain on the surface of the catalyst, and the other products leave the reactor with the oil–hydrogen stream. The reactor effluent is cooled before separating the oil from the hydrogen-rich gas. The oil is stripped of any remaining hydrogen sulphide and light ends in a stripper. The hydrogen gas is treated to remove hydrogen sulphide and ammonia, and then recycled to the reactor.

The sulphur removal is typically > 99.9 % and the nitrogen removal is typically >99.5%, failing to meet these specifications the treated diesel-product is not suitable for sale and loses its margin. 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 0.99 M$/yr. These improvements need to come from improved maintenance strategies on the HDS reactor, the recycle gas compressor, the heat exchangers and the pumps in the unit.

The unit is subdivided into 3 functional Systems: reaction section, stripper section, gas recycle section.

Function of the Reaction section is to treat 17.2t/h diesel and remove 99.9% sulphur.

Functional failures of the Reaction System causing production loss are:

• Less than 17.2t/h diesel throughput of the System
• Unable to heat the product to 350 °C at 17.2t/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 17.2t/h diesel and remove 99.9% sulphur/H2S.

Functional failures of the Stripper System causing production loss are:

• Less than 17.2t/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 main Equipment in the HDS unit relevant for the RCM study are distributed of these three Systems:

Based on these 17 Equipment about 40 Failure Modes are estimated for IMS-RCM Analysis – taking about 10 days in Analysis time.

An IMS-RBI study for the HDS unit is on the year plan for next year with implementation completed on the following year. 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 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 units 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.

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 availability 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.

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 centred 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)