Contact Section - Blowers - Specification
April 25, 2004

Introduction Capacity         Normal Design Flow         Minimum Flow         Overdesign Corrosion Allowance Critical Speeds Materials of Construction Tests Applicable Standards

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Introduction

The main blower is one of the single most expensive pieces of equipment in a sulphuric acid plant.  To ensure that the main blower will operate trouble free and at the intended capacity it must be specified properly from the very beginning in terms of the process and mechanical design criteria.

Capacity

The capacity of the blower is determined by the amount of gas or air that must be handled to achieve the production rate of the plant.   The capacity of the blower should allow for the normal design flow as well as the minimum flow requirements.

Normal Design Flow

The normal design flow will be the flow rate required to achieve the plant production requirements. 

Minimum Flow

The minimum flow through the blower is generally set by the preheat requirements of the acid plant.  In some cases, the process requirements of the plant may dictate the minimum flow but this is unusual.   During preheating air is heated in the preheat system and is used to preheat the converter and catalyst to the minimum strike temperature of the catalyst.  The flow requirements are set by the time required to preheat the plant which is generally 24 to 36 hours.  To achieve this requirement a flow rate of 30 to 40% of the normal design flow is required.

Overdesign

An overdesign will generally be specified for the blower as a safety margin to ensure the plant can achieve its design production rate.  Typically, and overdesign of 5% of the normal design flow is specified.   In some cases the overdesign may be as high as 10% depending on design requirements specified by the plant operator.

Corrosion Allowance

A corrosion allowance of 3 mm (1/8”) is generally specified for the casing.

Critical Speeds

API Standard 617 defines critical speeds as:

“Critical speeds correspond to resonant frequencies of the rotor-bearing support system.  The basic identification of critical speeds is made from the natural frequency of the system and of the forcing phenomena.  If the frequency of any harmonic component of a periodic forcing phenomena is equal to, or approximates, the frequency of any mode of rotor vibration, a condition of resonance is may exist; if resonance exists at a finite speed, that speed is called a critical speed.”

Blowers should be designed so that there are no critical speeds within the normal operating range of the blower.

Materials of Construction

The materials of construction for the blower should be suitable for the service conditions and gas being handled. 

Casing (Castings)	Grey Cast Iron (GG25) (DIN 0.6025) Cast Iron (A40 40B) Cast Iron (A278 Class 40)
Shaft	17CrNiMo6 Alloy Steel
Shaft Sleeve	316 SS
Shaft Bushing	X35CrMo17 (DIN 1.4122) A511 Mt 440 A
Impeller - Blades	AISI 4130 StE 460 (1.8905/A572 Grade 65) X 5 CrNi 13 4 (DIN 1.4313)
Impeller - Hub	StE 500 (1.8907/A514 Type A-P) X 5 CrNi 13 4 (DIN 1.4313)
Inlet Guide Vanes - Housing	Grey Cast Iron (GG25) (DIN 0.6025) Cast Iron (A40 40B)
Inlet Guide Vanes - Vanes	X 6CrNiMoTi 17122 (DIN 1.4571) Cast 316L SS
Base Frame	RSt 37-2 (DIN 1.0038) A284 Grade B
Oil Reservoir	RSt 37-2 (DIN 1.0038) A284 Grade B
Oil Piping - Upstream of Filters	Carbon Steel (A53 Grade A)
Oil Piping - Downstream of Filters	X6CrNiTi1810/X6CrNiMoTi 17122 (1.4541/1.4571) A276 Type 321/AISI Type 316 304 SS

Tests

The following tests are generally performed during the manufacture and testing of the blower.

• Hydrostatic pressure test of the compressor casing

• Dynamic balance of the rotating element at reduced speed

• Mechanical running test with shop driver at full speed and reduced load

• Impeller overspeed test during mechanical running test at 115% speed

Impeller undergoing dynamic balancing

Impeller being lowered into a vacuum overspeed pit for overspeed test

Applicable Standards

The following standards are generally applied to the design, fabrication and testing of a blower for acid plant service.

AGMA 6011-H98	Association of Gear Manufacturers Specification for High Speed Helical Gear Units (formerly AGMA 421)
API Standard 614	Lubrication, Shaft-Sealing, and Control Oil Systems for Special-Purpose Applications
API Standard 617	Centrifugal Compressors for General Refinery Service
API Standard 670	Noncontacting Vibration and Axial Position Monitoring Systems
ISO 5389	Turbocompressor Performance Test Code (replaces VDI 2045)
ISO 8011	Turbocompressor Design and Construction
ISO 10817	Shaft Vibration for Industrial Turbosets – Measurement and Evaluation (replaces VDI 2059)
DIN ISO 1940	Shaft Vibration for Industrial Turbosets – Balancing (replaces VDI 2060)
DIN 4312	Steam Turbine Design and Construction
VDI 2056	Verein Deutscher Ingenieure Evaluation Criteria for Mechanical Oscillations of Machines