It was my luck to
participate in Northwest Redwater Refinery project. As introduced in previous
post, LC Finer unit deals with heavy residue from Vacuum and Distillation Unit. The unique process conditions and commodity
properties, made us facing challenges daily. Clamps connectors were one of
them.
Because of the
high temperature and high pressure process, a lot piping systems were fall into
#1500 and #2500 rating. Meanwhile, as H2S and hydrogen exposure conditions,
alloyed and stainless steels had to be applied. In this case, conventional
connections such as flanges, could be extremely heavy, bulky and expensive.
After a careful study, Grayloc connectors were selected over flanges in classes
of 1500 and 2500. It was a brilliant and brave decision, which demonstrated the
engineering capabilities of Fluor.
Clamp Connector Components
The clamp
connector consists of three types of components, the seal ring, the two hubs
and the clamp assembly, normally consisting of two clamp halves and four bolts.
Advantages of Clamp Connectors
Clamp connectors
offer considerable advantages opposed to conventional connections such as
flanges, especially in high pressure piping systems.
Pressure Energized,
Metal-to-Metal Sealing
Clamp connectors
do not rely on the conventional flange gaskets and correctly tensioned bolts to
maintain the integrity of the seal. As the hubs are drawn together by the clamp
assembly, the seal ring lips deflect against the inner sealing surfaces of the
hubs. This deflection elastically loads the lips of the seal ring against the
inner sealing surface of the hub, forming a self-energized seal.
Size & Weight
Up to 75% lighter
and significantly smaller than a comparable ANSI or API flange.
360° Orientation
Radial bolting on
clamp connector allows 360° orientation around the pipe.
Less Bolting, No
Alignment
Only 4 bolts to
tighten rather than up to 24 on a conventional flange, with no bolt holes to
align.
Minimize
Maintenance Downtime
Much quicker
make-up and release time
Leak free: reduce
emissions
A proven track
record of gas tight leak free operation when correctly installed. The pressure
energized bore seal withstands temperature and load variations, which
ultimately can cause conventional flanges to leak.
Lower bolt torque
Typically 50%
less than a ring joint flange. Sealing integrity is as well unaffected by
over-tensioning bolts.
Reusable seal rings
Unlike
spiral-wound gaskets, reusable metal seal rings give greater
flexibility for hydro-testing and commissioning prior to service.
Disadvantages of Clamp Connectors
Flanges are commonly
used. ASME 16.5 for flanges and flanged fittings, ASME 16.47 for large diameter
flanges regulate all aspects of flange manufacture and uses. On the other hand,
clamp connectors are available at several manufacturers and these manufacturers
will have different types even in the same rating class. Although designed
to meet ANSI B 16.5, ASME VIII DIV 2, API 6A/Q1 and other design codes, clamp
connectors are not standardized dimensionally under ASME code, therefore
introduce a risk for incompatibility during piping installation, if
administered improperly during the design.
1.
During NWR project, Grayloc was client’s favorite. All hub ended valves
and fittings were tagged and logged as specialty items. It put massive work load
on material management.
2.
SPEC breaks:
There are many
concerns at SPEC breaks, such as pipe wall thickness and materials. Since the wall
thicknesses and thus internal diameter for each nominal pipe size could be
different per application, the internal diameter of the seal rings may vary.
Therefore, different hubs and seal rings may be required for identical Nominal
Pipe Sizes.
In our practice,
for the SPEC break connectors, each component was selected, described and tagged. Material engineers worked out a long list on
all SPEC break Grayloc connectors. Then CAD support team put them into
SmartPlant 3D database. When modeling, designers had to pick the right connector
from the drop down list by tag number, to ensure bill of material is correct.
3.
Nozzles on vessels:
Since hub faced
nozzles are not standard, a set of clamp connector had to be ordered. One piece
of hub was sent to the vessel vendor for the nozzle and the rest parts were shipped
to fabrication shop for the pipe spool.
4.
Break spool require extra pair of hubs:
Unlike flange
gaskets, seal rings are “T” shaped and much wider. When clamps are
disassembled, the hub faced item can’t be removed easily as flanged. In most
cases, an extra set of clamp connector has to be added at 90 degrees direction
as a removable spool. Showing as below:
5.
Some items require special design due to the uniqueness of clamp
connectors:
Blind Hubs - for
blinding of a Pipeline / Equipment
Blind or
Spectacle Seal rings - for isolating sections of pipe or in-line blind service
Restriction Seal
rings - to reduce Pressure / Flow
Insulating
Connectors - to prevent galvanic corrosion or to provide electrical isolation
Orifice Seal
rings - for Flow measurement (with integral or replaceable Orifice Plates)
Orifice Unions -
With Pressure taps for Flow measurement with Orifice Seal rings
Butt weld Hubs
for Orifice Unions shall be of streamline bore design and are to be provided
with Orifice tap holes of which the centerline on the inside is located 25.4 mm
(1 inch) from the Orifice Plate plane.
Manufacturers
provided different solutions. But the construction and operation of them are
much more difficult than using flanges.
6.
Stress calculation
Most
Manufacturers include pressure / temperature rating tables for the materials
applicable in their catalogue. These pressure ratings may exceed the ASME 16.5
table 2 rating values for flanges, but do not include allowances for External
Loads (Forces / Bending Moments) acting on the Connection due to Pipe Stresses.
Although it is explained that the clamped connection can withstand severe external
loading without leaking, it may seem upon calculation, that the selected
components are not able to withstand the combined load of internal pressure and
external Loads.
As our stress
lead Junaid Khan mentioned: The biggest drawback is low Vendor allowable
external forces and moments. On NWR these hub clamps were used on high
temperature piping, complying with recommended loads necessitated the use of
spring supports and obtaining required flexibility was a pain. The purchase
documents should specify minimum acceptable external loads. ASME Section
VIII, Div-1, Appendix 24 “Design Rules for Clamp Connectors” and API
publication” Bending Moment Calculations for special API Hub/Clamp
combinations” provide a basis for establishing reference values. On NWR this
was not done and the piping team was pretty much left at the mercy of Grayloc,
the Client’s preferred Vendor.
Above is the observation from a
designer’s point of view. There might be other concerns on project management,
procurement, delivery and installment sides. Clamp connectors usage is
spreading, the disadvantages can be avoid with more experience and regulations on
them.
