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What Is Deadweight? | Piping Engineering Glossary
What Is Deadweight?
In piping engineering and process engineering, deadweight is the permanent gravity load of a piping system: the weight of the pipe, its contents, insulation, cladding, valves, flanges, and other attached components. It acts continuously for the life of the system and is one of the basic loads every pipe support must carry.
What Deadweight Includes
Deadweight is usually calculated for the operating condition, using the actual fluid density. Gas lines may have very little content weight in service but must still be checked for water weight during hydrostatic testing. Concentrated weights, such as large valves and their actuators, often govern support placement because they create high local bending.
In a pipe stress analysis, deadweight is analyzed as its own load case and combined with pressure to give the sustained load stresses that are checked against code limits. The same analysis reports the load each support must carry. Deadweight results also show where the pipe sags most between supports, which matters for drainage.
Applications
Support Placement
Deadweight analysis determines where pipe supports are needed and how far apart they can be without overstressing the pipe. Heavy valves and vertical runs usually need supports placed close to them.
Spring Support Design
Deadweight loads at hot and cold conditions are used to select each variable spring support, and the installed cold load is set from these calculations.
Equipment Nozzle Protection
Supporting deadweight close to pumps and vessels reduces nozzle loading that could misalign shafts or overstress connections. Spring or rigid supports near the nozzle often carry most of this weight.
Benefits
Clear, Predictable Load
Unlike thermal or wind loads, deadweight is constant and well defined, making it straightforward to calculate. Weights can be taken from piping models, vendor data, and standard tables.
Basis for Support Design
Accurate deadweight values allow supports, structures, and foundations to be sized correctly.
Prevents Sag and Overstress
Properly supporting deadweight keeps pipes straight, drainable, and within sustained stress limits. This also protects flanged joints from bending loads that can cause leaks.
Limitations
Weight Data Must Be Accurate
Underestimating insulation, valve, or refractory weights leads to overloaded supports and unconservative stress results. Refractory-lined and heavily insulated lines are especially sensitive to these errors.
Conditions Change
Changes in fluid density, insulation upgrades, or added equipment can increase deadweight after the original design. Any significant change should trigger a check of the affected supports.
Not the Only Load
Deadweight must be combined with occasional loads such as wind and earthquake, as well as thermal effects, to get a complete picture.
FAQ
Is deadweight the same as sustained load?
Not exactly. Deadweight is the gravity load itself, while sustained load stresses combine the effect of deadweight with internal pressure and other long-term loads.
Should hydrotest weight be included in deadweight?
Hydrotest weight is usually checked as a separate case. If it governs, temporary supports may be added for the test rather than oversizing permanent supports.
How is deadweight shown in stress software?
It is typically labeled as the weight case, often designated W, and combined with pressure and thermal cases according to the code.
Is a deadweight tester related to deadweight loads?
No. A deadweight tester is a calibration instrument that uses precise weights to generate known pressures, and it has nothing to do with piping gravity loads.
Reference
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