On-demand Control System for Dust Collecting compatible with NFPA
Overview
This post outlines the requirements of the National Fire Protection Association’s (NFPA) Standards 664 and 652 concerning on-demand dust collection control systems, generally applicable to industrial exhaust ventilation systems.
Ecogate Systems, when properly designed, installed, and calibrated, meet the NFPA 664 and NFPA 652 requirements for control systems. Note, these standards are intricate, with control systems constituting only a minor part. Crucially, Ecogate systems monitor and regulate minimum air volume and velocity throughout the duct system, a key component of these standards.
UPDATED: NFPA 660 Consolidated
The National Fire Protection Association (NFPA) has recently published the inaugural draft of NFPA 660, the Standard for Combustible Dusts. This development represents a significant paradigm shift in the approach facilities must adopt to identify hazards and implement protective measures against combustible dust risks.
NFPA 660 aims to provide clarity and cohesion by amalgamating six pre-existing standards — NFPA 61 (Food), NFPA 484 (Metals), NFPA 652 (Fundamentals), NFPA 654 (Chemical), and NFPA 664 (Wood) — into a singular, comprehensive standard. This integration is anticipated to streamline the process of managing combustible dust hazards.
The new standard is poised to offer a dual approach: it will set forth both fundamental and industry-specific guidelines for effective combustible dust management. This includes conducting thorough dust hazard analyses and endorsing best practices for safeguarding facilities against dust-related explosions and fires.
NFPA Dust Standards Today
NFPA 664 | NFPA 652
Pressure-regulated control systems, usually a pressure transmitter connected to a Variable Frequency Drive and set to maintain constant pressure in the main duct, cannot alone meet NFPA 664 or 652 standards. These systems only regulate pressure, not air velocity or volume.
For instance, if you block off the main duct, the pressure-regulated system will continue to regulate to the set pressure, but there is no airflow, no minimum transport velocity. This means the system is on the left side of the fan curve with zero airflow.
As you might already know, according to NFPA Standard 652, facilities handling combustible dust must have completed Dust Hazard Analyses (DHAs) by September 7, 2020. You can freely access these standards at nfpa.org after creating an account.
A code becomes law when a municipal jurisdiction adopts it or includes it in an adopted code. You can determine whether these standards are adopted at your location by consulting your Authority Having Jurisdiction (AHJ). Alternatively, visit www.nfpa.org/Codes-andStandards and use the "CodeFinder". Some states, like Washington, extended the effective date for NFPA 652 code to February 1, 2021.
Below, we outline ten key requirements of NFPA-664 and NFPA-652 for dust collection systems, covering aspects such as design, air volume specification, system capacity, airflow monitoring, manual gate usage, system start and stop, maintaining minimum transport air velocities, warnings, and system design changes.
#1. Dust collection system design
NFPA 664-20 Guidelines:
Section 8.2.2.3.1.6: This section emphasizes the importance of maintaining the minimum system velocity, especially when using dampers or gates for individual equipment. It states that the operation of these components must not lead to a reduction in system velocity below the designed minimum.
Section 8.2.2.5: This part specifies the requirements for fans or blowers. It mandates that these components should be of a type and capacity that's adequate to maintain the necessary rate of airflow throughout all parts of the system.
NFPA 652-19 Recommendations:
Appendix A.9.3.3.3: This appendix addresses systems with variable-speed fans, particularly those added to existing dust collection setups. It advises a redesign of the duct system to meet specific requirements. One critical aspect to consider is the potential for significant pressure drops in smaller main ducts at full usage. Therefore, the fan sizing should accommodate both the minimum and maximum use conditions.
2. Specification of Required Air Volume for Each Point of Use & Dust Collector
NFPA 664-20 Guidelines:
Section 8.2.2.1.4: This section mandates that every part of the dust collecting system must be sized to meet or exceed the minimum air velocity and volume required for effective material transport through the ducting into the collection equipment.
Section 8.2.2.6 - Dust Collectors: It specifies that the system must include collection equipment with the necessary size and capacity to maintain required airflow and efficiently separate wood dust from the air before it is exhausted.
Section 8.2.2.4.2: This part focuses on ensuring that each collection point in the system is designed to achieve the minimum velocity required for capturing, controlling, and containing dust. It also notes that the minimum air volume should be properly documented (as per 8.2.2.4.3).
NFPA 652-19 Recommendations:
General Requirement: It is required to specify the necessary air volume for each point of use and the minimum velocity for each branch line and duct section between these points and the Air Material Separator (AMS) or dust collector.
Appendix A.9.3.3.3.1: Emphasizes the importance of maintaining a minimum air volume flow at each dust collection source point (hood) for optimal design.
Appendix A.9.3.3.3.2: Stresses the need for documenting the hood's minimum design value to facilitate field testing.
3. System Capacity
NFPA 664-20 Guidelines:
Section 8.2.2.3.1.5 A: This section requires the system's capacity to be calculated based on all hoods and openings connected to the system. These must either be open or equipped with means to ensure the minimum transport velocity, as outlined in Section 8.2.2.2.1.4. This standard dictates that every section of the dust collecting system must be sized to meet the minimum air velocity and volume requirements for effective material collection and transport.
Section 8.2.2.3.1.5 B: It specifies that floor sweeps, which are normally closed and only opened occasionally, are excluded from the airflow calculations for the system.
NFPA 652-19 Recommendations:
General Requirement: The fan package and Air Material Separator (AMS) or dust collector used in the system must be compatible with the full performance requirements, from all sources being open to the minimum sources open. The section warns that incorrect selection of these components can result in a failure to maintain the required duct velocities.
4. Monitoring Minimum Airflow at Each Drop/Branch
NFPA 664-20 Guidelines:
Not Addressed in this Standard.
NFPA 652-19 Recommendations:
General Requirement: Monitoring mechanisms should be implemented at each drop/branch to ensure that the minimum design airflow is consistently maintained when the branch is open.
Appendix A.9.3.3.3.2: This appendix highlights the necessity of documenting the minimum design airflow value, a requirement that facilitates effective field-testing.
5. Use of Manually Controlled Gates
NFPA 664-20 Guidelines:
Section 8.2.2.1.3: This section mandates that once system airflow is properly balanced, any additions or modifications such as extra pickup points, duct changes, or alterations in balancing damper settings should not be made without verifying that the rest of the system maintains adequate capture and conveying air velocities for their intended function.
NFPA 652-19 Recommendations:
Appendix A.9.3.3.3: It advises against the use of manual slide or "blast" gates. Their use is discouraged because they can lead to uncontrolled modifications in flow volumes for individual lines and the entire system, potentially causing imbalances. Proper design methodologies are recommended to inherently ensure minimum airflows and duct velocities, negating the need for manual slide or blast gates.
6. System Start
NFPA 664-20 Guidelines:
Section 8.2.2.2.2 - Startup: Pneumatic conveying, dust collection, and centralized vacuum cleaning systems must be designed to achieve and maintain the design air velocity before material is introduced into the system.
NFPA 652-19 Recommendations:
General Requirement: All gates should be open at system startup.
7. System Stop
NFPA 664-20 Guidelines:
Section 8.2.2.2.3.1: These systems must maintain design air velocity during a normal shutdown process until all material is purged from the system. This does not apply to emergency shutdowns.
NFPA 652-19 Recommendations:
Not Addressed in this Standard.
8. Maintaining Transport Air Velocities in the Entire Duct System
NFPA 664-20 Guidelines:
Section 8.2.2.4.2: The system must be designed to achieve the minimum velocity required for dust capture, control, and containment at each collection point. Minimum air volume should be documented (8.2.2.4.3).
NFPA 652-19 Recommendations:
General Requirements:
(3) The design must ensure required velocity is maintained in all open branches and duct sections under all conditions.
(4) The controller should automatically adjust to maintain minimum air velocity in all sub-branches and the main duct.
9. Warning When a System Fails to Provide the Required Performance
NFPA 664-20 Guidelines:
Not Addressed in this Standard.
NFPA 652-19 Recommendations:
General Requirement (7): Alarms should be installed to alert personnel when the system fails to meet required performance standards.
10. Changes to Dust Collection Systems
NFPA 664-20 Guidelines:
Section 8.2.2.1.3: Any additions or modifications to the system, such as additional pickup points or duct modifications, must not be made without ensuring that the rest of the system maintains adequate air velocities for their intended function.
NFPA 652-19 Recommendations:
Appendix A.9.3.3.3.3: Modifying or disconnecting a branch line can lead to imbalances and insufficient airflows in the system. The use of manual gates is discouraged.
Appendix A.9.3.3.3.4: Adding branch lines for new dust sources to an existing system can reduce air volumes and duct velocities in existing portions of the system.
Notes
This article references the revisions NFPA 664-2020 and NFPA 652-2019 of the NFPA standards.
Key Elements for Systems with Variable-Speed Fans: Outlined in the NFPA standards are critical design considerations for systems incorporating variable-speed fans. These guidelines ensure compliance with safety and efficiency standards, especially in scenarios of full system usage where smaller main ducts might encounter significant pressure drops. To address these challenges, the following elements are essential in the design:
(1) Specification of Air Volumes and Velocities: Clearly specify the required air volume for each point of use and the minimum velocity for each branch line and duct section, leading up to the Air Material Separator (AMS) or dust collector.
(2) Monitoring at Each Drop/Branch: Install monitoring systems at each drop/branch to ensure the maintenance of minimum design airflow when the branch is in operation.
(3) Maintenance of Required Velocity: Design the system to consistently maintain the required velocity in all open branches and duct sections under various usage conditions.
(4) Automatic Control Adjustments: Incorporate controllers that automatically open additional points of use or balance air dampers as needed to maintain minimum air velocity in all sub-branches and the main duct.
(5) Startup Procedure: Ensure that all gates are open at system startup for optimal performance.
(6) Compatibility of Fan Package and AMS: Select fan packages and AMS units that are compatible with the full range of performance requirements of the system, from all sources being open to only minimum sources open, to avoid failure in maintaining necessary duct velocities.
(7) Alarm Systems: Implement alarm systems to alert personnel when the system fails to meet required performance standards.
These guidelines are instrumental in ensuring the efficiency, safety, and compliance of dust collection systems with variable-speed fans.
For further inquiries regarding how Ecogate Systems address these requirements, feel free to contact us. We may cover this topic in more depth in a future article.
Author: Ales Litomisky
