3B1 Chapter 5 - High-Pressure Boiler Fittings Study Guide

High-Pressure Boiler Fittings: Overview and Classifications

High-pressure boiler fittings are critical components for the safe operation, maintenance, and monitoring of steam generation systems. These fittings are categorized based on their function, such as level indication, pressure relief, and blowdown management.

Water Level Gauges

Water level gauges are essential for determining the actual water level within the boiler drum. Various designs are utilized based on pressure requirements and visibility needs:

• Tubular Type: Basic design for lower pressure applications.

• Reflection Type: Uses light reflection to differentiate between steam and water.

• Transparent Type: Uses clear glass on both sides to view the water directly.

• Multi-Port Type (Bull’s Eye): Uses individual port assemblies for high-pressure durability.

• Protector Shields: Vital for operator safety in case of glass failure.

• Accessory Components: Includes Steam cocks, Drain cocks, and Water cocks for isolation and maintenance.

Steam Drum Internals

Drum internals are categorized into three distinct classes based on their function within the boiler system:

1. First Class: Devices dedicated to removing moisture and impurities from the steam (e.g., Steam separators).

2. Second Class: Components that distribute feedwater uniformly within the drum to prevent thermal shock (e.g., Internal feed pipes).

3. Third Class: Systems providing continuous blowdown to manage dissolved solids.

Key Components and Steam Separation

• Demister/Scrubber: Removes fine moisture droplets.

• Continuous Blowdown Line (CBD): Collects water with high concentrations of dissolved solids, typically located several centimeters below the operating water level.

• Downcomers: Conduits that carry cooler water from the drum to the lower headers.

• Risers: Tubes where steam-water mixtures rise back into the drum.

• Centrifugal/Cyclone Separators: Use high centrifugal force to separate water from the steam-water mixture entering from the risers.

• Chemical Dosing Line: Introduces treatment chemicals directly into the drum.

• Steam Washers: Spray approximately $5\%$ of boiler feedwater into outgoing steam to absorb gaseous impurities like silica.

Code References and Regulatory Compliance

The design, construction, installation, and usage of high-pressure boiler fittings are governed by the following regulatory bodies and standards:

• ASME Section I: Specific rules for the construction of power boilers.

• ASME Section VII: Guidelines for the care and operation of power boilers.

• CSA B-51: Canadian standards for boilers and pressure vessels.

Note: Operating procedures provided in study materials are for informational purposes. Always consult site-specific standard operating procedures (SOPs) and equipment-specific manuals.

Objective 1: High-Pressure Relief Valves

ASME uses the umbrella term Pressure Relief Valve (PRV) to describe safety valves, safety relief valves, and relief valves.

HP Relief Valve Design

High-pressure relief valves, typically constructed with cast steel bodies, are suitable for:

• Maximum Pressure: Up to $3450\,kPa$

• Maximum Temperature: Up to $400^\circ C$

Mechanical Components

• Valve Disc (D): Held against the seat by the spring force.

• Spring (J): Provides the downward closing force.

• Adjusting Nut (L): Sits at the top of the spring; used to increase or decrease compression, thereby changing the set pressure.

• Lock Nut (N): Secures the adjusting nut in place once the setting is established.

• Cap (B): Protects the internal adjustment mechanism.

Installation Requirements (ASME Section I, PG-71.2 & 71.3)

1. Direct Connection: The valve must be connected to the boiler independent of any other connection.

2. Proximity: Attached as close as possible to the boiler.

3. Orientation: Must be installed in an upright position with the spindle vertical.

4. No Obstructions: No valve of any type shall be placed between the safety valve and the boiler, nor on the discharge pipe.

5. Discharge Pipe Sizing: The cross-sectional area of the discharge pipe must not be less than the full area of the valve outlet and must be as short and straight as possible.

6. Drainage: Gravity drains must be provided for condensate accumulation. Each valve must have an open gravity drain from the casing located below the level of the valve seat.

7. Support: Risers must be supported independently from overhead to eliminate mechanical stress on the valve.

8. Drip Pan and Elbow: Used with sufficient clearance to allow for thermal expansion. Drains must be checked for ice or blockages prior to startup but should not be inspected while the system is under pressure in case the PRV opens.

Operation of High-Pressure Relief Valves

The fundamental physics of these valves relies on the relationship: $Force = Pressure \times Area$

The Pop Action and Huddling Chamber

Pop-type PRVs feature a disc with a lip or skirt.

• Huddling Chamber (Pop Chamber): The area below the disc lip.

• Process: When steam pressure reaches the set point, the disc lifts slightly. This exposes the additional surface area of the lip to steam pressure. Because the effective area increases suddenly, the upward force increases substantially ($F = P \times A$), causing the valve to "pop" wide open immediately.

• Duration: The valve remains open until the boiler pressure drops below the set popping pressure.

Pressure Relief Valve Blowdown

Blowdown is defined as the difference between the pressure at which the relief valve opens and the pressure at which it closes.

ASME Section I PG-72.1 Requirements

The valve must be designed to operate with a minimum blowdown of:

• $15\,kPa\, (2\,psi)$, OR

• $2\%$ of the set pressure (Whichever is greater)

Example: If a valve opens at $1000\,kPa$ and closes at $970\,kPa$, the blowdown is $30\,kPa$, or $3\%$.

Adjusting the Blowdown Ring

The blowdown ring (adjusting ring) varies the size of the escape port opening to the huddling chamber:

• Screwing the Ring Upwards: Decreases the opening of the escape port. This directs more steam against the lip, increasing lifting force. Result: Blowdown increases (lower reseat pressure).

• Screwing the Ring Downwards: Increases the opening of the escape port. Less steam contacts the lip. Result: Blowdown decreases (higher reseat pressure).

• Adjustment Procedure: Remove the lock screw, insert a screwdriver to move the ring, and then re-lock and seal the screw.

Pressure Relief Valve Sizing (ASME PG-67.1)

• Minimum: Each boiler must have at least one safety valve.

• Two or More Valves Required if:     * The boiler water heating surface exceeds $47\,m^2$.     * An electric boiler has a power input exceeding $1100\,kW$.

Objective 2: Pressure Gauges

ASME Code Section I Requirements (PG-60.6)

• Attachment: Must be attached to the steam space or the water column.

• Isolation: Gauges must be fitted with an isolation valve.

• Protection: The connection must include a siphon or similar device to prevent live steam from entering and damaging the gauge internals.

• Test Connection: Every boiler must have a separate connection of at least $6\,mm\, (1/4\text{ NPS})$ for the exclusive use of a Boiler Inspector’s test gauge.

• Graduation: The dial must be graduated to at least $1\frac{1}{2}$ times the safety valve set pressure. Ideally, the range is twice the Maximum Allowable Working Pressure (MAWP).

Forced Flow Steam Generators (PG-60.2)

For generators without a fixed steam space or water line, gauges are required at:

1. The boiler or superheater outlet.

2. The boiler or economizer inlet.

3. Upstream of any isolation valve between sections of the heat-absorbing surface.

Objective 3: Water Columns and Gauge Glasses

Accurate water level monitoring is critical; low-water level combined with safety interlock failure is a leading cause of boiler incidents.

The Water Column

A water column is a reservoir attached to the boiler drum. Its purposes include:

• Damping: Stabilizes water level by dampening agitation for easier reading.

• Sediment Collection: Collects impurities away from the gauge glass.

• Control Housing: Provides a mounting point for high/low level alarms and probes.

Code Requirements (ASME PG-60.1)

• Pressure Threshold: Boilers with a MAWP above $400\,psi$ must have two gauge glasses.

• Remote Indicators: One of the two required glasses can be replaced by two independent remote water level indicators.

• Visibility: The lowest visible water level in the glass must be at least $2\,in\, (50.8\,mm)$ above the lowest permissible water level determined by the manufacturer.

Gauge Glass Types

1. Flat Gauge Glass: Uses thick glass plates bolted in steel housings with thin mica sheets to protect the glass from etching at high pressures.

2. Bull’s Eye Gauge Glass: Uses multiple individual port assemblies. Suitable for high pressures up to $20.7\,MPa\, (3000\,psi)$.

3. Bicolour Gauge Glass: Uses refraction to provide high visibility. Green light passes through the water reflected by the screen; red light passes through the steam space.

Ancillary Features

• Illumination: Lighting assemblies must be sufficient to penetrate filter glass.

• Quick Shut-Off Devices: Chain valve arrangements allow operators to isolate a cracked gauge glass from a safe distance (e.g., from the deck below).

Objective 4: Testing and Maintenance

Testing Procedures for Water Columns and Gauge Glasses

1. Proving the Water Connection

• Close steam line isolation valves (1 and 2).

• Ensure water column drain is closed (5).

• Keep water line open (3 and 4).

• Open gauge glass drain (6).

• Observe flow; then return to normal (close 6, open 1 and 2).

2. Proving the Steam Connection

• Close water line isolation valves (3 and 4).

• Ensure water column drain is closed (5).

• Keep steam line open (1 and 2).

• Open gauge glass drain (6).

• Observe flow; then return to normal (close 6, open 3 and 4).

3. Proving the Water Column Drain Line

• Close valves 3, 4, and 2 to isolate the gauge glass.

• Ensure gauge glass drain (6) is closed.

• Ensure steam valve (1) is open.

• Open column drain valve (5).

• Return to operating position (close 5, open 3, 4, and 2).

Gauge Glass Maintenance

• Corrosion: High alkalinity causes thinning of the glass. Silica dissolution by condensate further weakens the material. Effects increase as pH rises.

• Mechanical Stress: Misaligned fittings are a common cause of failure.

Objective 5: Low-Water Fuel Cut-Offs (LWCO)

The LWCO protects the boiler by shutting off fuel to burners if the water drops to a "low-low" level.

LWCO Types

1. Float Type:     * Uses a mercury bulb switch.     * A two-wire switch starts the feedwater pump.     * A three-wire switch activates the shutdown and alarm if the level continues to drop.

2. Probe Type:     * Relies on the electrical conductivity of water.     * Shortest Probes: Start/Stop feedwater pump or trigger High-High alarms.     * Longest Probe: Completes the circuit to the Low-Low level switch; when water drops below this probe, the circuit is broken, and the fuel valve is de-energized.

Testing Guidelines

• Frequency: Weekly or monthly inspection; annual overhaul/dismantling during outages.

• ASME Section VII (101.4.2.3): Avoid tripping the boiler during live tests. The LWCO must be set to trip before the water level disappears from the gauge glass.

Objective 6: Stop Valves

Every steam outlet (except safety valves/reheaters/superheaters) must have a stop valve as close to the boiler as possible.

Outside-Screw-and-Yoke (OS&Y) Design

If the outlet exceeds $50\,mm$ pipe size, the valve must be the OS&Y rising spindle type.

• Advantage: Provides visual confirmation of the valve position from a distance.

• Protection: The threaded spindle is outside the body, protecting it from corrosion and allowing for lubrication.

Common Stop Valve Types

1. Gate Stop Valves: OS&Y rising spindle. Uses locking lugs and seal welds for high-pressure bonnet integrity.

2. Angle Stop Valves: A glove valve variant with a bottom inlet and side outlet. Includes a drain valve above the disc to remove condensate before opening.

3. Globe Stop Valves: Handwheel rises with the spindle. Excellent for throttling and flow control.

Non-Return Stop Valves (NRV)

NRVs (automatic stop-and-check valves) are required when multiple boilers connect to a common header.

• Function: Prevents header steam from backflowing into a lower-pressure boiler. Facilitates bringing a boiler online automatically when drum pressure exceeds header pressure.

• Types: Globe, Y-type, Angle, and Spring-Cushioned (which uses a spring to prevent chattering during small pressure fluctuations).

Objectives 7, 8 & 9: Blowoff and Blowdown Systems

Definitions

• Blowdown: Refers to PRV pressure differences OR the removal of solids ($CBD$ or $IBD$).

• CBD (Continuous/Surface Blowdown): Removes solids just below the water level.

• IBD (Bottom Blowdown): Removes mud and sediment from the lowest point of the boiler.

• Blowoff Water: The water removed by these processes, piped to a blowoff tank.

Blowoff Valve Arrangements (ASME PG-58.5)

Two valves in series are required for blowoff connections.

• Slow-Opening Valve: Requires at least five full turns ($360^\circ$ each) of the handwheel.

• Quick-Opening Valve: Opens/closes via a lever moved through a $90^\circ$ arc.

• Configuration: The quick-opening (sealing/guard) valve must be closest to the boiler. The slow-opening (sacrificial) valve is used for the actual blowing down to prevent water hammer.

Blowoff Valve Types

1. Sliding Disc: Quick-opening; moves a disc off the opening.

2. Seatless Sliding Plunger: Slow-opening; plunger ports coincide with body ports.

3. Seat and Disc: Slow-opening; uses hard materials like Stellite or Monel to resist erosion.

4. Combination: Two valves in one body (integral seat inlet and seatless plunger discharge).

Pressure Rating Limits for Valves

• Up to $450\,psi$: Seatless valves acceptable.

• Up to $600\,psi$: Hard-seat blowing valve and seatless sealing valve combination.

• Up to $2500\,psi$: Two hard-seated valves required.

Objective 10: Manual Blowdown Procedures

Sequencing Rules (ASME Section VII, 101.4.2.7)

Note on Double Seatless Valves: Opening the valve nearest the boiler last prevents compressing trapped water between the plungers.

Objective 11: Steam Drum Internals and Separators

Steam Separation Devices

1. Dry Pipe Separators: Steam enters holes in the top, changes direction, and sheds water droplets.

2. Cyclone Separators: Use centrifugal force to spin water out of the steam-water mixture.

3. Steam Scrubbers (Dryers): Secondary stage of separation using Chevron V-shape corrugated plates. Corrugation maximizes surface contact to catch fine droplets.

Feedwater and Chemical Systems

• Internal Feed Pipe: Distributes water uniformly to avoid temperature shock. Located in the lower-temperature water space to encourage circulation into downcomers.

• Baffles: Girth baffles and riser baffles separate turbulent rising steam/water from quiet areas near downcomers.