Designed for cyclic pressure spikes and high temperatures.
Must exceed Maximum Allowable Working Pressure (MAWP).
Relevant to design of piping and associated valves.
Blowdown vs. Blow Off
Definitions
Blow Off: Intermittent process removing accumulated solids (e.g., sludge, debris) from the boiler, specifically from the bottom of the mud drum.
Blowdown: Continuous process aimed at controlling water quality, especially the amount of dissolved compounds in the water. Based on water tests, particularly conductivity levels.
Key Differences
Blow Off: Occurs at various intervals to remove bulk amounts of water. Typically relates to solids removal.
Blowdown: Based on need for continuous testing and monitoring of water quality.
Bottom Blow Off Valves
Valve Types
Slow Opening Valve
Definition: Requires five full 360-degree turns to fully open or close.
Designated as throttle valves.
Must be hard-seated or have a slotted seat.
Quick Opening Valve
Definition: Requires a quarter turn (90-degree arc) to transition from fully open to fully closed.
Not designed for throttling.
Types of Slow Opening Valves
Hard Seated Globe Valve
Flow direction: 90-degree turn.
Features wear-resistant materials for plug and seat given harsh conditions of shock service.
Y-Type Globe Valve
Design: Stem at an angle to reduce pressure drop and minimize flashing and cavitation.
Improves performance against sediment accumulation.
Seatless Slow Opening Valve
No plug or seat; utilizes a plunger that slides up/down inside the valve body.
Handwheel activates the plunger position.
Contains packing compression springs to prevent leaks.
Quick Opening Valve Mechanism
Composed of polished discs with a spring in between to ensure sealing.
Effectively slices through sludge and debris for improved operation.
Designed for rapid opening and closing, located closest to boiler for optimal seal maintenance.
Design Considerations
At least one slow opening valve is required per ASME B31 code for boiler operations.
Design of the blow-off vessel must comply with ASME Section VIII, ensuring safety even if not pressurized.
Blow Off Vessel Functionality
Purpose
Cools and depressurizes blow-off water for safe disposal to the sewer system.
Adheres to local sewer system regulations (e.g., maximum permissible temperature).
Regulations and Compliance
Maximum temperature for sewer discharge is 65°C according to CSA B51.
Local jurisdiction dictates compliance; typically lower temperatures take precedence.
Cooling Process
Utilizes a temperature control valve controlled based on a temperature probe:
Introduces city water supply to mix with blow-off water, rapidly cooling to below 60°C.
Importance of cooling: Protects infrastructure and marine life from thermal impact.
Mechanism of Operation
Water enters the blow-off tank at an angle to promote swirling, enhancing cooling and sediment suspension.
Maintains water level through a siphon effect via a small outlet tube with an anti-siphon hole to prevent vacuum conditions.
Best Practices for Blow Off Operations
Ensure operational awareness—confirm the correct boiler is being serviced.
Monitor the drum water level visually or have assistance if visibility is obstructed.
Open/close valves slowly to prevent shock conditions; only perform blow off on one boiler at a time.
For simultaneous operations: Follow the rule of "inside, outside, outside, inside" for the sequence of valve operations.
Special considerations for seatless valves: Operate in the reverse sequence to avoid trapping fluids.
Conclusion on Blow Off Procedures
Blow off frequency should be managed to control sludge buildup without excessive water waste.
Additional controls may include managing water levels for startup or shutdown conditions to avoid high/low water risks.
Regular inspections by ABSA are required to ensure compliance and operational integrity of pressure vessels involved in blow off.