UG-99(b) Hydrotest: Static Head Is Location, Not Margin

For a vertical vessel tested vertically with the pressure gauge at the top, calculate the top test pressure from P = 1.3 × MAWP × Sa/S; no test-liquid static head exists at that elevation. Pressure at every lower point equals the top pressure plus rho × g × h. Static head is therefore an elevation correction for local pressure, not a blanket addition to every hydrotest calculation.

Pressure Symptoms by Gauge Location

The number that matters is the pressure at the component being evaluated. A gauge reports pressure only at its own elevation, while the vessel contains a pressure gradient created by the test liquid.

Hydrostatic Pressure Mechanism

Hydrostatic pressure rises with depth according to Delta P = rho × g × h, where rho is the test-liquid density, g is gravitational acceleration, and h is the vertical distance between two points. Vessel orientation changes h; it does not change the basic relationship.

For a gauge at the top of a filled vertical vessel, the pressure at the bottom is:

P_bottom = P_top + rho × g × H

If the top target is calculated from the supplied UG-99(b) expression, the bottom pressure becomes:

P_bottom = (1.3 × MAWP × Sa/S) + (rho × g × H)

This distinction matters because moving static head inside the multiplier produces a different result:

P_alternative = 1.3 × (MAWP + rho × g × H) × Sa/S

The difference between these expressions is (1.3 × Sa/S - 1) × rho × g × H. That difference is not a change in fluid physics. It comes from applying the test factor to the head term. Use that form only when the governing calculation procedure explicitly defines the pressure basis at that lower elevation before applying the factor.

Pressure Datum and Required Quantities

Every pressure value needs an elevation datum. Labels such as “hydrotest pressure” or “MAWP” are incomplete for a tall vessel unless the drawing, calculation, or test procedure identifies where the value applies.

The lowest component MAWP cannot be used safely as a location-free shortcut. Components occupy different elevations and may use different materials or allowable stresses. Compare the actual local test pressure with the permissible pressure for each component at its elevation.

Vertical-Test Calculation Procedure

1. Establish the reference point. Record the elevation at which the approved MAWP or design-pressure basis applies. Record the intended gauge elevation separately.
2. Calculate the UG-99(b) reference pressure. Using the notation supplied for the vessel, calculate P_reference = 1.3 × MAWP × Sa/S. Take the stress values and temperatures from the governing calculations and applicable code edition.
3. Locate the gauge. For a top-mounted gauge at the same reference elevation, use P_gauge,target = P_reference. No liquid-head correction is needed at that point.
4. Translate the target when the gauge is lower. If the gauge is a vertical distance h below the reference point, use P_gauge,target = P_reference + rho × g × h. If it is above the reference point, subtract the corresponding head.
5. Calculate local component pressure. For each shell course, head, nozzle, flange, closure, and other pressure-retaining item, calculate P_local = P_reference + rho × g × h_local, using vertical distance from the reference elevation.
6. Check the limiting component. Compare every local pressure with the permitted test pressure or test-temperature rating for that component. The governing location may be at the bottom because pressure is highest there, but material properties, geometry, or component rating can make another location limiting.
7. Set the operating target. Put the target gauge pressure, gauge elevation, test-fluid density basis, vessel orientation, and maximum permitted pressure in the written test procedure.

Verification During the Test

Before pressurization, verify that the vessel is completely filled and that trapped gas has been vented from high points. Compressed gas stores far more energy than liquid at the same pressure and changes the risk of a failure.

During filling, a second gauge at another elevation provides a direct head check. With both gauges exposed to the same static liquid column and corrected for calibration, their pressure difference should correspond to rho × g × Delta h. A mismatch points to an incorrect elevation, different zero reference, trapped gas, an isolated connection, or gauge error.

At the target pressure, verify four items:

1. The controlling gauge reaches the target assigned to its elevation.
2. The calculated pressure at the top meets the required UG-99(b) reference pressure.
3. The calculated pressure at every lower component remains within its permitted test condition.
4. The test record identifies gauge serial data, calibration status, elevation, orientation, liquid, temperature basis, target pressure, and achieved pressure.

Pressure loss by itself does not identify leakage. Liquid temperature change, vessel expansion, hose movement, pump isolation, and trapped gas can move the reading. Stabilize the system and evaluate the pressure together with temperature and a physical inspection.

Horizontal Testing of a Vertical Vessel

A vertical vessel tested horizontally has a smaller elevation difference between its top reference point and many components. At the same gauge pressure, its lower components therefore see less static head than they would during a vertical test.

If the approved test basis requires the horizontal test to reproduce the pressure that the installed bottom would experience during a vertical test, calculate the target from local-pressure equality:

P_horizontal,gauge + rho × g × h_horizontal
= P_vertical,top,target + rho × g × H_vertical

Solve for the horizontal gauge target using the actual gauge and bottom elevations. The correction is the difference between the vertical and horizontal static heads, referenced to the same component. This orientation adjustment is not an automatic instruction to raise pressure: first check whether the resulting pressure would exceed the permitted test condition anywhere in the horizontally supported vessel.

Horizontal supports can also load the shell differently from installed vertical supports. The pressure calculation and the temporary-support assessment are separate checks.

Recurring Calculation Pitfalls

• Adding full vessel head to a top gauge target. The top gauge has no liquid column above it. Adding full head there raises pressure at every point and may overpressure the bottom.
• Ignoring head at a bottom gauge. A bottom gauge naturally displays the reference pressure plus liquid head. Setting it only to the top target leaves the top below target.
• Multiplying static head without defining the datum. Adding head to MAWP before applying 1.3 × Sa/S is not algebraically equivalent to adding physical head after calculating the top target.
• Treating gauge position as component pressure. Translate the reading by rho × g × Delta h before comparing it with a component limit.
• Using vessel length instead of vertical height. Hydrostatic head depends on vertical elevation difference, not shell length or distance measured along an inclined vessel.
• Using an assumed fluid density. Read density for the actual test liquid and temperature basis used in the procedure.
• Checking only the nameplate value. Local component ratings and test-temperature allowable stresses can govern before the overall vessel value does.

Test Documentation and Decision Control

The test package should show a single elevation sketch with the pressure reference, gauge, vessel top, vessel bottom, and limiting components. Beside each point, list elevation, calculated static head, required local pressure, and permitted local pressure. This converts a disputed “add head or not” question into an auditable pressure balance.

If calculations use design pressure instead of MAWP, identify that choice and its approval basis. Keep the selected code edition, stress-ratio source, material temperatures, orientation, and component limits attached to the test record; otherwise a later reviewer cannot reproduce the target.

Frequently Asked Questions