OISD-STD-125
Revised Edition August 2007
INSPECTION AND MAINTENANCE
OF
MECHANICAL SEALS
OISD-STANDARD-125
First Edition, July 1990
Amended Edition, August 1999
Revised Edition, August 2007
Oil Industry Safety Directorate
Government of India
Ministry of Petroleum & Natural Gas
(i)
OISD - STD- 125
First Edition, July 1990
Amended edition, August 1999
Amended Edition, August 2007
INSPECTION AND MAINTENANCE
OF
MECHANICAL SEALS
Prepared by:
Functional Committee
Oil Industry Safety Directorate
7th FLOOR, “NEW DELHI HOUSE”
27, BARAKHAMBA ROAD
NEW DELHI-110 001
(ii)
NOTE
OISD publications are prepared for use in the oil and gas industry under Ministry of Petroleum and Natural Gas. These are the property of Ministry of Petroleum and Natural Gas and shall not be reproduced or copied and loaned or exhibited to others without written consent from OISD.
Though every effort has been made to assure the accuracy and reliability of the data contained in these documents, OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting from their use.
These documents are intended only to supplement and not to replace the prevailing statutory requirements.
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FOREWORD
The oil industry in India is 100 years old. As such a variety of practices have been in vogue because of collaboration/association with different foreign companies and governments. Standardisation in design philosophies and operating and maintenance practices at a national level were hardly in existence. This, coupled with feed back from some serious accidents that occurred in the recent past in India and abroad, emphasised the need of the industry to review the existing state of art in designing, operating and maintaining oil and gas installations.
With this in view, the then Ministry of Petroleum & Natural Gas in 1986, constituted a State Council assisted by Oil Industry Safety Directorate (OISD) staffed from within the industry, in formulating and implementing a series of self-regulatory measures aimed at removing obsolescence, standardising and upgrading the existing standards to ensure safer operations. Accordingly, OISD constituted a number of Functional Committees of experts nominated from the industry to draw up standards and guidelines on various subjects.
The present document on " Inspection And Maintenance
Of Mechanical Seals " was prepared by Functional Committee
constituted for total review of OISD standards on Rotary
Equipment.
This document is based on the accumulated knowledge and experience of Industry members various manuals, national and international codes of practices. This document is meant to be used as a supplement and not as a replacement for existing codes, standards and manufacturers' recommendations. It is hoped that provision of this document if implemented objectively may go a long way to improve safety and reduce accident in the oil and gas industry. Suggestions for amendments, if any, to this document should be addressed to:
This document in no way supersedes the statutory regulations of CCE, Factory Inspectorate, or any other statutory body which shall be followed as applicable.
Suggestions for amendments to this document should be addressed to
The Coordinator,
Oil Industry Safety Directorate,
7th Floor, New Delhi House
27, Barakhamba Road
New Delhi – 110 001
(iv)
LIST OF FUNCTIONAL COMMITTEE MEMBERS
S.N / Name / Organisation / Position in Committee01 / Mr. D.K. Puri / Reliance Industries Limited / Leader
02 / Mr. S.K. Chatterjee / Hindustan Petroleum Corporation Limited / Member
03 / Mr. P. Veerabhadra Rao / Hindustan Petroleum Corporation Limited / Member
04 / Mr. T.V. Venkateswaran / Reliance Industries Limited / Member
05 / Mr. A.K. Dash / Indian Oil Corporation Limited / Member
06 / Mr. Deepak Prabhakar / Mangalore Refinery and Petrochemicals Ltd. / Member
07 / Mr. R.C. Agarwal / Bharat Petroleum Corporation Limited / Member
08 / Mr. K Ravi / Kochi Refineries Limited / Member
09 / Mr. Shamsher Singh / Oil Industry Safety Directorate / Member
Coordinator
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CONTENTS
S.N Chapter Page No.
1.0 Introduction 01
2.0 Scope 01
3.0 Definitions 01
4.0 Safety Considerations for Selection of Seals 02
5.0 Commissioning, Operation and Maintenance 03
6.0 Inspection, Installation and Testing 04
7.0 Non Contacting (Dry Gas) Seals 04
8.0 Failure Analysis 06
9.0 References 06
10.0 Annexure-I 07
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Inspection & Maintenance
of
Mechanical Seals
1.0 Introduction
Mechanical Seals are vital component of rotating equipment like pumps and compressors. The proper selection, operation and maintenance of mechanical seals is critical for safety and environment. This document has been prepared, considering the importance of mechanical seals in the overall safety of the installation. The rotary equipment covered by this document are Centrifugal Pumps & Compressors.
2.0 Scope
This standard covers the minimum requirements for selection, operation, maintenance & failure analysis of Mechanical Seals and Sealing Systems installed on rotating equipment in hydrocarbon industry. The document covers both contact and non-contact type mechanical seals.
3.0 Definitions
3.1 Balanced Seal
A mechanical seal arrangement whereby the effect of the hydraulic pressure in the seal chamber, on the seal face closing forces, has been modified through seal design.
3.2 Barrier Fluid
A fluid, which is introduced between dual mechanical seals to completely, isolate the pump process liquid from the environment. Pressure of the barrier fluid is always higher than the process pressure being sealed.
3.3 Buffer Fluid
A fluid used as a lubricant or buffer between dual mechanical seals. The fluid is always at a pressure lower than the pump process pressure being sealed.
3.4 Cartridge Seal
A completely self-contained unit (including seal gland, sleeve and mating rings), which is pre-assembled and preset before installation.
3.5 Dual Mechanical Seal
A seal arrangement using more than one seal in the same seal chamber in any orientation, which can utilize either a pressurized barrier fluid or a non-pressurized buffer fluid.
3.6 Mating Ring
A disk or ring-shaped member, mounted either on shaft sleeve or in a housing, which provides the primary seal when in proximity to the face of an axially adjustable face seal assembly.
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3.7 Seal Ring
The seal face that contacts the mating ring. It is flexibly mounted using springs or bellows.
3.8 Shaft Sleeve
A cylindrical sleeve placed over the shaft to protect it from wear and corrosion.
4.0 Safety Considerations for Selection of Seals and Sealing Systems
4.1 Selection criterion for Mechanical Seals shall include;
a) Focus on the parameters that can result in unsafe condition during operation
b) Fire hazard
c) Health and environment concerns
d) Provisions in the basic design that shall prevent occurrence of unsafe conditions.
4.2 List of equipment handling such fluids, which can have adverse effect on health safety and environment in the case of leakage, shall be maintained.
4.3 For each of such application, seals and sealing systems shall be selected in consultation with the seal manufacturer and as per guidelines given in the national/ international standards such as API 682, OISD & ISO.
4.4 The factors to be considered for selection of mechanical seals are as below:
a) Media
b) Temperature
c) Pressure
4.4.1 Media
Fluid containing Toxic, Carcinogenic and Corrosive Fluids require seals that have enhanced safety features, like Tandem Seals, Double/ Dual Seals etc.
Dual seals shall be applied for following services:
a. Liquids containing hydrogen sulphide in concentrations above 600 Mg/kg.
b. Liquids consisting of or containing lethal substances. Examples of lethal substances include but are not limited to: HF acid, phenol, concentrated sulphuric or nitric acid.
c. Hydrocarbon services of butane (C4) or lighter.
d. Liquids consisting of or containing very toxic substances like benzene, toluene, MEK, ethylene oxide etc.
4.4.2 Temperature
Fluids having potential for auto Ignition/ Vaporization at operating temperatures and cryogenic services shall have enhanced safety features.
Dual seals with barrier fluid shall be applied for Hydrocarbon liquids at an operating temperature above their auto-ignition temperature. For example,
a) Vacuum Residue above 250 deg C
b) Vacuum Gas Oil above 257 deg C
c) Motor Spirit and gasoline above 280 deg C
d) Kerosene above 210 deg C
4.4.3 Pressure
Dual seals shall be used for hydrocarbon liquids with a vapour pressure more than or equal to 5 bar (abs) at the operating temperature.
4.5 Review of the existing installations should be carried out and necessary corrective steps to comply with the requirements in clauses 4.4 taken to the extent feasible.
5.0 Commissioning, Operation, Monitoring
Procedures shall be in place to ensure that seals and sealing systems are properly commissioned, operated and monitored so as to prevent failures, resulting in unsafe conditions. The procedures shall be as per the guidelines below:
5.1 Commissioning
Commissioning of seals shall include the first commissioning of the system as well as the re-commissioning after repairs. Procedures shall cover the following;
a) Flushing of the main process and seal piping to ensure elimination of debris/ dirt etc.
b) Use of temporary fine strainers to arrest entry of debris/ dirt into the seal
c) Proper venting of the equipment, seals and sealing systems to safe locations
d) Ensuring completeness of auxiliary systems. e.g.- Flush & Quench Plans, Monitoring & Control Devices, Cooling & Cleaning systems.
e) Ensuring proper functioning of the auxiliary systems by suitable means like temperature or flow measurement.
5.2 Operation & Monitoring
Proper operation of equipment as well as sealing system is essential to ensure that there are no catastrophic damages to mechanical seals resulting in unsafe conditions. It should be ensured that proper procedures are in place to ensure that the mechanical seals are operated properly. These procedures shall include the following;
a) Ensuring proper operation of equipment as recommended by the OEM/ relevant OISD standards.
b) Proper venting of the equipment, seals and sealing systems to safe location
c) Ensuring proper functioning of the auxiliary systems by suitable parameters like temperature/ temperature gradients of flush, and cooling water circuit or flow measurement.
d) Monitoring temperature, pressure and level of the buffer fluid in seal pot.
e) Visual inspection for any leakage from seal/ seal systems.
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A typical list of checks that can be done on seals is given in Annexure-1.
6.0 Inspections, Installation and Testing
The following aspects of inspection, installation and testing of seals should be considered, but not limited to, during repair/ reconditioning of seals.
6.1 Inspection
6.1.1. All components of the seal shall be inspected by appropriate methods to ensure proper quality.
6.1.2. Seal faces shall be inspected and may be re-used after lapping provided the surface flatness and finish meets the requirement.
6.1.3. Elastomers normally undergo degradation and shall be replaced with new ones.
6.1.4. Other components like sleeve, springs and gland plates shall be inspected and re-used if no damage is observed.
6.1.5. Ensure proper functioning of all trips and alarms for the seal system
6.1.6.
6.2 Installation
6.2.1. Ensure that all assembly/ installation of seals shall be done as per seal drawings.
6.2.2. Proper checks shall be done to ensure that the equipment condition is good and does not cause any failures of the seals.
6.2.3. The seal should be installed after the following checks;
a) Axial Movement
b) Radial/ Shaft Whip
c) Shaft Run-Out
d) Stuffing Box Square ness
e) Concentricity of the Stuffing Box.
6.2.4. Skilled and trained manpower shall be used for assembly/ installation of seals.
6.2 Testing
6.3.1. Procedures shall be in place to ensure that the mechanical seals installed in equipment are tested prior to taking in operation.
6.3.2. All seals after installation into the equipment shall be subject to a static pressure test to ensure safe operation.
7.0 Non-Contacting (Dry Gas) Seals
This section covers those procedures that are to be considered for the safe operation of non-contacting seals specifically compressor seals. These procedures are over and above the procedures mentioned in the earlier sections.
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7.1 Selection
7.1.1. Selection of dry gas seals should be done based on the pressure, temperature, service etc. for all the operating / static conditions.
7.1.2. Separation gas shall be provided, when required, to prevent gas from the seal assembly migrating into the bearing chamber and oil from bearing chamber migrating into the seal assembly. Separation gas should be instrument air or inert gas like Nitrogen.
7.1.3. The buffer gas system shall be capable of reliably supplying buffer gas at sufficient pressure and flow to positively prevent outward leakage of process gas under all operating and static conditions.
7.1.4. The seals should be designed for bi-directional rotation to avoid sudden seal failure due to reverse rotation, if reverse rotation is expected.
7.1.5. Seal control system should have adequate monitoring and safety features such as;
a) Duplex seal gas filter with differential pressure monitoring
b) Flow meter in the seal gas supply line to both the seals.
c) Control valve/ regulator for the buffer gas to the secondary seal
d) Flow meter in the leak-off gas line to flare
e) Pressure gauge and pressure transmitters in the flare line.
7.2 Commissioning
7.2.1 Operating procedure for dry gas seal system shall be prepared and the operating personnel shall be trained for the same.
7.2.1 Ensure trips and alarms for seal gas systems are set as per applicable OEM recommendations and shall be kept in line.
7.2.1 Static leak test for inboard seal leakage as per the procedure given in the seal manufacturer’s instruction manual shall be carried out. Any abnormality shall be analysed and resolved.
7.2.1 If an external source of seal gas is used, either permanently or temporarily, the gas shall be compatible with the process and with the materials of construction of the compressor and downstream equipment.
7.2.1 Before starting lube oil pumps, separation gas supply shall be ensured.
7.3 Operation & Monitoring
7.3.1 The supply of seal gas shall be monitored to each seal.
7.3.2 A differential pressure alarm/ trip should be provided to warn about loss of seal gas pressure.
7.3.3 The seal should not be operated if the leakage is greater than factory set shutdown trip levels.
7.3.4 The seal leakage should be continuously monitored and recorded; a trend of increasing leakage may give forewarning of a seal problem.