What supports have to do
A pipe filled with fluid can be surprisingly heavy, and the weight does not sit still. Supports have to carry the weight of the pipe, its contents, any insulation and any attached valves or instruments. They also have to hold the line in position, resist forces from pressure and flow, allow for thermal movement, and keep vibration from building up.
When supports are missing, too far apart or badly chosen, pipes sag, hold liquid in low spots, strain flanged joints and push on connected equipment. Over time, the damage shows up as leaks, cracked welds or pumps that will not stay aligned.
Dead weight and other loads
The load on a support comes from more than weight. Designers consider:
- Sustained load, the pipe, fluid and insulation weight.
- Pressure thrust, where pressure acts on bends and ends and tries to push them out.
- Thermal loads, from the pipe growing or shrinking against restraints.
- Occasional loads, such as wind, earthquake, or the shock from a pressure surge.
- Test loads, because a line filled with water for pressure testing is often heavier than in normal service, especially for gas or steam lines.
Support spacing depends on pipe size, material, wall thickness and contents. Plastic pipe, which is less stiff, typically needs closer support than steel, and its supports need to take account of its larger thermal movement. Specific spacing values come from codes and manufacturer data and are not rules to guess.
Types of supports
Support hardware falls into a few families:
- Rigid supports, such as pipe shoes on steel, saddles, clamps and rests. These carry weight from below.
- Hangers, which suspend the pipe from above using rods and clamps. A simple rod hanger swings slightly, allowing small movement.
- Spring hangers and supports, which carry a load while allowing vertical movement, useful where the pipe rises or falls as it heats and cools. Constant-force versions keep the carried load nearly steady across the range of movement.
- Sliding supports, with low-friction surfaces that allow the pipe to move along its length.
- Guides, which let the pipe move in one direction while preventing sideways motion.
- Anchors, which fix a point of the line completely.
- Snubbers and restraints, which allow slow movement but resist sudden motion, such as from shock or seismic events.
The choice of anchors and guides is closely linked to how the line grows, a theme covered in the discussion of thermal movement. Anchors in the wrong place can send large forces into equipment.
Keeping weight off equipment
A principle repeated across plant design is that piping should not hang its weight on pumps, compressors or vessel nozzles. Connected equipment typically has limits on the loads it can take at its connection points. Piping next to a pump is normally supported separately so the machine does not carry pipe load, and so the pump can be removed without the line sagging. This also protects alignment, which is a theme in articles about shaft alignment and flexible couplings. A strained pump casing can pull a shaft out of line.
How vibration develops
Vibration in a piping system can come from several sources:
- Rotating or reciprocating equipment, such as pumps and compressors, whose pulsations travel into the pipe.
- Flow effects, including turbulence, partly open valves, and two-phase flow where gas and liquid mix.
- Pressure surges, which can slam pipes sideways. The mechanism behind these is explained in the water hammer article.
- Cavitation, where vapor bubbles collapse, shaking the pump and pipe. This is discussed under pump cavitation.
- External excitation, from nearby machines, wind or vehicles.
If the natural frequency of a pipe span lines up with the frequency of a source, the pipe can resonate, and the movement grows. Long, unsupported spans and heavy attachments such as valves on small-bore branches are especially prone to resonance.
What vibration does to a piping system
Vibration works through fatigue: repeated small movements that eventually crack metal. Small-bore connections, welded branch points, instrument taps and drain or vent lines are common weak spots because they stick out from the main pipe and carry the weight of valves or gauges. Vibration also loosens bolts and clamps, wears support surfaces, fretting the pipe where it rubs, and damages instruments. A bolted flange that keeps leaking may be a symptom, not a cause, a point linked to flanged joint practice.
Ways of reducing it
Approaches usually start with finding the source. Once the cause is known, common remedies include:
- Adding or repositioning supports to stiffen spans and shift natural frequencies.
- Bracing small-bore branches and supporting heavy valves.
- Using pulsation dampeners or properly sized accumulators near reciprocating machines.
- Correcting cavitation or poor flow conditions at the source.
- Using flexible connections at equipment, where designed, to isolate vibration.
- Rubber or spring isolators and snubbers where appropriate.
Choosing the fix without understanding the cause can simply move the problem. For equipment-based sources, vibration analysis is the technique plants use to identify the frequencies involved.
Inspection and good habits
Walkdowns should check that supports are present and in contact, that spring hangers are within their travel range, that sliding supports are free and not corroded stuck, and that clamps and bolts are tight. Signs of trouble include bright rub marks, damaged coatings, bent hanger rods, cracked welds and pipes that visibly shake. Removed supports must be put back after maintenance.
Modifying supports on a live line can change the load paths, so changes should be reviewed by qualified people. This article is general information, not engineering advice.