This document discusses API 671 requirements for couplings used in gas compression applications and provides recommendations from TB Wood's. Key points include: - API 671 requires 5+ year minimum service life and higher service factors than normal selection criteria. TB Wood's recommends using standard selection criteria. - Transient/peak torques should be specified by the purchaser and validated with a torsional analysis. TB Wood's will use standard selection criteria. - Compliance with API 671 spacer retention and other requirements would require significant modifications from TB Wood's standard designs. Exceptions are recommended where possible to allow use of stock components for repairs. - Additional costs associated with API 671 compliance include drawings/documentation, engineering time,

1. DATE:August 21,2015 NO. OF PAGES: 3
TO: Ketul Shah - EMAIL: KShah2@Dresser-Rand.com
COMPANY: Dresser Rand
SUBJECT: API 671 4th
Edition - Couplings for Gas Compression Application
Here are notes that are applicable to low speed compressor applications. I have included the
relevant section in API 671 in (brackets). Paraphrased wording of the requirement is in
italics. Our comments are listed after that. I summarized the issues and cost drivers at the
end of the document. Let us know if you want us to specifically do any of the things I said we
would not do.
Our general guidance is to stay as close to standard design couplings as possible in order to
assure service parts availability in the future. Practical experience has been that once a unit
is installed, users don’t demand API compliance in a breakdown situation. This raises a real
question of why you should go to the extra expense of making the coupling initially comply.
There is greater risk by changing the torsional characteristics of the unit when you install
stock cast spacers to replace the higher-stiffness machined steel versions than there is to not
using the time-proven standard design.
General Application of API 671: (1 & 5.2) Flexible element couplings to be designed for
minimum service life 5 years in continuous and uninterrupted service in an equipment train
that is normally unspared and is critical to the continued operation of the installation.
Service Factor: (6.7) Minimum of 1.5 for flex disc couplings. We recommend much higher
service factors for engine driven screw compressors. Our normal selection criteria and
service factors should be used.
Transient/Peak Torques: (7.3) Purchaser will specify magnitude, nature and frequency of
such occurrences. Use standard selection criteria and validate with a torsional analysis.
Spacer Retention: (8.1.6) Unless otherwise agreed, the coupling design shall be such that
the spacer is retained in case of a complete flex element. Our bolting will do this to some
degree, but not completely. Modification for complete compliance will be a significant
modification. We will quote our standard type unless specifically requested otherwise.
Spacer Length: (8.3) Couplings must be of spacer type to allow space to maintain adjacent
bearings and seals without disturbing the equipment. 18” minimum DBSE is required. We
recommend using the standard DBSE for the coupling size selected to allow future use of
stock components for repairs. This is a specific exception to API 671. It should be noted
however that in meeting API requirements the torsional characteristics of a coupling initially
TB Wood’s Incorporated
2000 Clovis Barker Road
San Marcos, Texas
78666
Tel:
512-353-4000
ex.2244
FROM THE DESK OF
Arcadio Salas Jr.

2. TB Wood's Incorporated 2000 Clovis Barker Road, San Marcos, TX 78666
made to meet API 671 will be different from the standard product.
Tapered Bore Hubs: (8.6.2.6) Tapered bores will be checked using a gage supplied by the
customer which is matched to the specific shaft. The user will most likely force an exception
to this. Only Cooper-Superior is still in production with a tapered shaft as far as we know.
We have taper gages in stock which were made to their specifications and working
cooperatively with them.
Keyed Hubs: (8.6.3.4) Keyways will have radiused corners and hubs shall have tapped
puller holes. Both of these are special requirements.
Component Fits: (8.9.1) For couplings operating at 1800 RPM or less, fits are required to
meet only the balance tolerance. Our standard tolerances are adequate.
Nuts: (8.10.3) Deformed thread self-locking fasteners shall be used. We use nylon-inserted
lock nuts and we will take exception and provide the standard unless the customer
specifically requires compliance.
Spare Hardware: (8.10.7) A minimum of 2 spare coupling bolts/nuts shall be supplied with
each coupling.
Balance: (9.2) Component balance per method 1 is standard for couplings operating at
1800 RPM or less. We will supply with component balancing only.
Balance: (9.3.4) All hardware components to be individually weigh balanced.
Materials: (10.1 & 10.2) Materials shall be the manufacturer’s standard except as required
or prohibited by data sheets or the standards. Material grades and specific physical
properties will be identified to relevant industry standards. It is generally assumed that
castings are not desired or allowed for API 671 compliance even though it is not specifically
stated in the 4th
edition. Material testing requirements make castings impractical. We will
supply machined steel hubs, spacers and flywheel adapter plates.
Flex Elements: (10.5) Flexible elements to be corrosion resistant. Stocked 304SS to be
used unless otherwise specified.
Fasteners: (10.7) All fasteners shall be heat treated to grade 5 or better. Stocked hardware
is grade 8.
Inspection Records: (12.3.2) Shall be maintained for minimum of 5 years.
NDT Inspection: (12.3.4) All metallic torque transmitting components to be wet-mag or dye
penetrant inspected.
Welds: (12.3.5) NDT inspection required for all welds. Washers are not welded on to hubs
for GCH/GCF series

3. TB Wood's Incorporated 2000 Clovis Barker Road, San Marcos, TX 78666
Marking: (12.5.6) All couplings to be uniquely serialized.
Summary of items affecting cost and delivery to meet API 671 requirements with FSH/HSH
series couplings:
1. API 671 compliance costs
a. Drawings & Inspection Documentation
b. Engineering design time
c. Special project management resources.
d. Setup/Lot charges to allow specific material traceability
2. 18” DBSE if specified.
3. All Steel Construction, no castings allowed
4. Stainless flex discs
5. Special locknuts if specified
6. NDT testing
7. Shimming if specifically required with taper bore hubs
8. Balancing
Summary of potential service issues when API 671 couplings used.
1. No spare parts stock available
2. Possibly need to fabricate special length spacer
3. Provision for shimming with taper bore
4. Change of torsional properties if standard spares used for field replacement
5. Loose washers (welding not feasible) not included in standard repair kits