Steam Traps, Water Hammer, and Pipe Insulation Study Guide

Chapter 5: Steam Traps, Water Hammer, and Insulation

Introduction to Steam and Condensate Management

In industrial settings, steam serves as a primary driving force for mechanical power or as a medium for heating. Steam systems are designed to be robust and provide long service lives; however, they are susceptible to damage if not operated correctly. Steam condensate forms when steam transitions from its vapor state to a liquid state. This occurs during heating processes as steam transfers its latent heat energy to the product being heated or to the steam line itself.

Importance of Condensate Removal

Recovering and reusing condensate is vital for system efficiency. Continuous removal of condensate from steam lines is mandatory to avoid several issues:

Water Hammer: If condensate is not removed, it can be accelerated by steam to high velocities. When this moving liquid strikes immovable objects like pipe bends or valves, it creates massive forces that can rupture pipes and fittings.
Turbine Damage: The admission of moisture into turbines is highly undesirable. High-speed moisture droplets can strike and damage turbine blades.
Heat Exchanger Efficiency: Complete removal of condensate from the shells of steam heat exchangers is crucial to prevent poor temperature control and airlocking.
Corrosion and Pitting: Systems must remove air and carbon dioxide (\text{CO}_2) to prevent pitting and corrosion.
Airlocking: Air acts as an excellent insulator, impeding heat transfer in exchangers and preventing the movement of liquid within the condensate return system.

Steam and Condensate Return Systems

The design of steam and condensate systems must discourage the accumulation of condensate in unwanted locations. Properly sloped lines are essential to prevent stagnant condensate from lying in the piping.

Critical Locations for Removal Devices

Devices such as drain lines, traps, and separators must be installed at specific points where condensate is known to accumulate:

Upstream of connections to steam risers.
At the terminal ends of steam header mains.
Ahead of expansion joints and pipe bends.
At the inlets to steam valves and regulators.

System Components and Flow

A typical system functions as follows:

Steam enters a separator to remove entrained moisture.
Steam continues into the header and enters expansion loops.
Changes in steam flow may cause condensate to form at the inlet and outlet of these loops; steam traps are installed at these points.
A condensate tank collects the liquid for eventual reuse.

Steam Separators (Steam Purifiers)

Steam separators are installed in steam lines to remove moisture droplets and suspended impurities. They utilize centrifugal force and gravity to separate moisture through two primary designs:

Baffle-Type Separators

Mechanism: These utilize baffles to cause a sudden change in the direction of steam flow, which causes moisture to drop out of the vapor stream.
Efficiency: They are simple in design but possess low efficiency.
Discharge: Collected moisture at the bottom goes through a drain to a trap for discharge to the return system.

Centrifugal Steam Separators

Mechanism: These use centrifugal baffles to impart a whirling (spinning) motion to the steam. Moisture and impurities, being heavier than steam, are thrown outward toward the separator walls.
Features: Separated steam passes through a drain baffle to reduce the spinning motion. High velocity is required for effective separation.
Drawbacks: Higher steam velocities lead to higher pressure drops.

Mechanical Coalescing and Centrifugal Separators

Mechanism: These combine centrifugal principles with a coalescing element (demisting screen).
Process: Water droplets gather on the screens upstream of the centrifugal element and drop out. Any remaining moisture is removed in the subsequent centrifugal section.
Efficiency: This type offers the highest separation efficiency.

Steam Traps: Principles and Designs

Steam traps are automatic devices that filter out condensate and non-condensable gases without allowing steam to escape.

Operational Principles

Mechanical: Based on density changes (buoyancy).
Thermostatic: Based on temperature differences between steam and condensate.
Thermodynamic: Based on the velocity and heat energy differences between steam and condensate.

General Requirements for Steam Traps

Long life and dependable service.
Corrosion resistance.
Efficient venting of air and carbon dioxide.
Ability to operate against backpressure in return lines.
Ability to function despite scale and sediment.

Mechanical Steam Traps

Ball Float and Thermostatic (F&T) Trap

Operation: Condensate enters the trap, causing the float to rise and open the outlet valve. It includes a thermostatic bellows element to release air. When steam reaches the trap, the bellows expands to close the vent.
Advantages: Works with light or heavy loads; unaffected by pressure changes; no airlocking at startup.
Disadvantages: Susceptible to water hammer damage; bellows not suitable for superheated steam; not freeze-resistant for outdoor cold-weather use.

Inverted Bucket Trap

Operation: When the bucket is down, the valve is open. Entering steam collects in the top of the bucket, providing buoyancy to lift it and close the valve. Air and $\text{CO}_2$ escape through a small vent in the bucket into the trap body to be discharged first when the valve opens.
Advantages: Simple construction; handles superheated steam and water hammer.
Disadvantages: Susceptible to airlocking during startup (slow air discharge); not freeze-resistant.

Thermostatic Steam Traps

Bellows-Type Thermostatic Trap

Operation: A corrugated bellows is filled with a volatile liquid (e.g., alcohol). Steam vaporizes the liquid, increasing internal pressure to expand the bellows and close the valve. Cool condensate causes the bellows to contract and open the valve.
Advantages: Handles large amounts of condensate and air; self-draining (no freezing).
Disadvantages: Bellows susceptible to water hammer and corrosion; not for superheated steam.

Bimetallic Trap

Operation: Uses two dissimilar metal strips welded together that bend when heated. Cold condensate flows through the open valve; steam heat causes the strips to deflect and close the valve.
Advantages: Suitable for low to high pressures and superheated steam.
Disadvantages: Causes subcooling of condensate; modest response time; requires periodic calibration.

Liquid Expansion Trap

Operation: Uses a tube filled with special oil. Hot condensate/steam causes the oil to expand, pushing a plunger to close the valve. Cooling causes contraction and reopening.
Advantages: Adjustable condensate release temperature; handles water hammer via a relief spring.
Disadvantages: Tube susceptible to corrosion; requires a long rod for movement.

Thermodynamic Steam Traps

Impulse Trap

Operation: Uses a piston-type valve. Cool condensate lifts the valve. If condensate is at steam temperature, it flashes into steam in a control cylinder, creating pressure that forces the valve closed.

Controlled Disc Trap

Operation: The only moving part is a disc. Condensate/air forces the disc up. High-velocity steam traveling under the disc creates a pressure reduction (Bernoulli principle), while steam reaching the chamber above the disc exerts high pressure to force the disc down.
Advantages: Good for high pressure and superheated steam; handles water hammer/vibration; simple (one moving part).
Disadvantages: Low capacity; fails at low pressure differentials or high backpressure; prone to airlock.

Trap Selection, Sizing, and Capacity

Factors for Selection

Condensate capacity at startup vs. normal operation.
Condensate and steam header temperatures.
Pressure differential across the trap.
Location (indoor vs. outdoor/freezing conditions).

Safety Factors for Sizing

Calculated condensate loads are typically increased by a safety factor to ensure capacity during pressure drops or condition changes:

Safety Factor of 2: For traps located between the boiler and the end of the header.
Safety Factor of 3: For traps located at the terminal end of the main header.

Calculation: Automatic Warm-Up Load

To calculate the condensate amount ( $C$ ) produced during the warming of a steel pipe: $C = \frac{0.494 \times M \times (t_2 - t_1)}{L}$

$0.494$ : Specific heat of steel pipe in $\text{kJ/kg} \cdot ^{\circ}\text{C}$
$M$ : Total mass of pipe in $\text{kg}$
$t_2$ : Final temperature of pipe ( $^{\circ}\text{C}$ , usually saturation temp of steam)
$t_1$ : Initial temperature of pipe ( $^{\circ}\text{C}$ )
$L$ : Latent heat of steam in $\text{kJ/kg}$

To find the load in $\text{kg/h}$ , divide $C$ by warm-up minutes and multiply by $60$ .

Calculation: Manual Warm-Up (Radiation Loss)

In manual warm-up, traps only handle radiation losses. The formula is: $C = \frac{A \times U \times (t_1 - t_2) \times E}{L}$

$A$ : External area of pipe ( $\text{m}^2$ )
$U$ : Heat loss from uninsulated pipe ( $\text{kJ/m}^2 \cdot ^{\circ}\text{C}$ )
$t_1$ : Steam temperature ( $^{\circ}\text{C}$ )
$t_2$ : Air temperature ( $^{\circ}\text{C}$ )
$L$ : Latent heat at operating pressure ( $\text{kJ/kg}$ )
$E$ : Insulation efficiency (expressed as $1 - \text{efficiency percentage}$ )

Linear Interpolation

Since steam tables and condensate load tables (Table 1 and 2) provide discrete values, interpolation is used for exact pressures/temperatures: $y = y_1 + (x - x_1) \times \frac{y_2 - y_1}{x_2 - x_1}$

Steam Trap Installation and Maintenance

Installation Requirements

Must be accessible, close to, and below the drip point.
Must follow the correct flow direction (arrow on body).
Equipped with isolation valves and a strainer on the upstream side.
Includes a test valve on the downstream side for visual checks.
Use check valves when multiple traps drain into a common header.

Commissioning Procedure

Blow the main header at full pressure via low point drains to remove dirt/scale.
Close trap isolation valves.
Remove trap and install a cap on the strainer.
Open inlet valve and partially open strainer blowoff to ensure flow.
Reinstall trap, clean strainer, and crack open inlet valve to fill trap.
Fully open outlet gate valve.
Follow plant-specific PPE and safety protocols.

Testing and Maintenance

Visual: Use test valves or sight glasses to check for continuous vs. intermittent discharge.
Temperature: Use thermometers/pyrometers to check the differential (not always reliable near saturation).
Sound: Use stethoscopes or ultrasonic devices (whistling, clicking, or quiet operation).
Annual Maintenance: Dismantle, clean, and inspect for corrosion. Replace worn parts (floats, bellows, seats). If a trap fails open, it wastes energy; if it fails closed, it causes water hammer.

Water Hammer

Water hammer refers to pressure surges or banging noises caused by the sudden deceleration of liquid or the collapse of steam pockets.

Types of Water Hammer

Condensate-Induced: Occurs when a steam bubble is enclosed by cooler condensate. The bubble collapses, creating a low-pressure void. Water rushes into this void from both sides, and the collision creates a massive pressure surge.
- Factors: Large temperature differences and large pipe diameters increase severity.
Flow-Induced: Occurs when a valve is closed too quickly, causing water velocity to drop and pressure to spike. A similar effect occurs when a pump stops and a check valve slams shut.
Steam Flow-Induced: Occurs when steam picks up a "slug" of condensate and drives it down the line at steam velocity. The slug hits a bend or valve, causing a surge of several thousand $\text{kPa}$ .

Prevention of Water Hammer

Properly slope steam lines toward drip stations.
Install drip traps every $90\,\text{m}$ to $150\,\text{m}$ and ahead of all valves, regulators, and risers.
Use Y-strainers with horizontally mounted screens.
Use gravity drainage for equipment with modulating regulators.
Startup Procedure:
1. Open all low-point drains fully.
2. Crack open the bypass (or main valve if no bypass) and listen for flow.
3. Monitor drains for condensate, then steam.
4. Close drains only when dry steam issues from them.
5. Gradually open the main valve as pressure rises.

Pipe Insulation

Purposes of Insulation

Retain heat and reduce heat loss to surrounding air.
Reduce condensation in steam lines.
Prevent personnel injury from contact with hot pipes.
Prevent "sweating" and external corrosion on cold pipes.

Temperature Categories

Cryogenic: Below $-75\,^{\circ}\text{C}$ .
Thermal: $-75\,^{\circ}\text{C}$ to $815\,^{\circ}\text{C}$ (subdivided into Low, Intermediate, and High).
Refractory: Above $815\,^{\circ}\text{C}$ .

Material Science and Calculations

Thermal Conductivity ( $k$ or $K$ ): The measure of heat transmitted through a material ( $\text{W/m} \cdot ^{\circ}\text{C}$ ). Lower value means better insulation.
Energy Transfer Equation: $Q = \frac{K \times A \times t \times \Delta T}{d}$
R-Value: Represents resistance to heat flow. High R-value is better. $R = \frac{\text{thickness}}{K}$ .
Pipe Insulation R-Value: Use equivalent thickness due to different surface areas: $\text{Eq. Thickness} = r_2 \times \ln(\frac{r_2}{r_1})$ .

Industrial Materials

Calcium Silicate: $38\,^{\circ}\text{C}$ to $650\,^{\circ}\text{C}$ . Granular lime and silica.
Glass Fibre: Up to $538\,^{\circ}\text{C}$ .
Mineral Fibre (Rock/Slag Wool): Up to $1040\,^{\circ}\text{C}$ .
Cellular Glass: $-273\,^{\circ}\text{C}$ to $200\,^{\circ}\text{C}$ ; poor impact resistance.
Elastomeric: Foamed resins, up to $104\,^{\circ}\text{C}$ .
Refractory Fibre: Alumina and silica; up to $1650\,^{\circ}\text{C}$ .
Insulating Cement: Good for irregular surfaces; Vermiculite up to $1038\,^{\circ}\text{C}$ .

Application Methods

Wrap and Clad: Insulation wrapped and covered in aluminum or stainless steel for weather and mechanical protection.
Insulated Blankets: Removable for maintenance; good for expansion joints but may allow external corrosion if not tight.
Covers and Boxes: Used for flanges, traps, and valves requiring frequent access.