Guidelines for rapid mixing devices

IS 7090 - Scope Key Points & Tables

Scope Summary:

• Covers mechanical and hydraulic mixing devices for water treatment.
• Defines detention time, velocity gradient, and power input for mixers.
• Includes specifications for hydraulic jump rapid mixing.

Key Table: Mechanical Mixing Devices (Clause 3.1.3)

Note: Power values are at 30°C water temperature.

Hydraulic Jump (Clause 5.1) Specifications:

• Velocity before jump: 3 to 3.5 m/s
• Head loss ≥ 300 mm at design flow
• Provides rapid mixing without mechanical units

Hydraulic Mixing Channel (Clause 1.5):

• Baffle angle: 40° to 90° to channel wall
• Minimum velocity at baffle: 1.5 m/s
• Channel walls: brick/stone masonry or RCC, smooth finish
• Minimum freeboard: 150 mm

This standard guides design for effective mixing in water treatment, ensuring proper energy input and flow conditions for coagulation and rapid mixing.

IS 7090 - Key Definitions & Specifications

1. Definitions (Clause 2.0)

• Standard terms related to rapid mixing in water treatment.
• Includes mechanical mixing devices, hydraulic jump, motor types, etc.

2. Mechanical Mixing Devices (Clause 3.1.3, Table 1)

• Note: Power calculations assume water temperature 30°C.

3. Hydraulic Jump (Clause 5.1)

• A rapid mixing method without mechanical units.
• Water velocity: 3 to 3.5 m/s in a sloping flume.
• Head loss at design flow: ≥ 300 mm.
• Kinetic energy dissipated in a pool ensures turbulence and mixing.

4. Motor Specifications (Clause 3.2.6)

• For vane/propeller mixers:
  • Totally enclosed fan-cooled (TEFC) motors, IS:325-1978.
  • Cooling method IC 41 (IS:6362-1971).
  • Protection: IP 54 or better (IS:4691-1985).

Summary Diagram: Flash Mixer Layout

flowchart LR
    RW(Raw Water Inlet Main) --> FM(Flash Mixer Chamber)
    FM --> CP(Chemical Dosing Pipe)
    FM --> DP(Delivery from Flash Mixer)
    DP --> SB(Strainer Box)
    SB --> IV(Isolating Valve)
    IV --> PUMP(Pump Suction)

This concise overview covers key definitions, power input data, hydraulic jump criteria, and motor specs per IS 709

IS 7090: Key Design & Construction Specifications

1. Hydraulic Mixing (Clause 1.5)

• Baffle angle: 40° to 90° with channel wall.
• Minimum velocity: 1.5 m/s while negotiating the baffle.
• Materials: Main channel walls - brick masonry, stone masonry, or RCC, finished smooth to avoid weed growth.
• Baffle: Concrete or brick, similarly finished.
• Freeboard: Minimum 150 mm above water surface.

2. Mechanical Mixing Devices (Clause 3.1.3)

Power values based on 30°C water temperature.

3. Hydraulic Jump (Clause 5.1)

• Velocity before jump: 3 to 3.5 m/s.
• Headloss: Minimum 300 mm at design flow.
• Purpose: Rapid mixing via turbulence without mechanical units.

Summary Diagram: Hydraulic Mixing Channel Setup

flowchart LR
    A[Inlet Water] --> B[Channel Wall]
    B --> C[Baffle (40°-90° angle)]
    C --> D[Velocity ≥ 1.5 m/s]
    D --> E[Outlet]
    style B fill:#f9f,stroke:#333,stroke-width:2px
    style C fill:#bbf,stroke:#333,stroke-width:2px

Note: Use smooth finishes on masonry/concrete surfaces to prevent weed growth and ensure hydraulic efficiency.

IS 7090: Mechanical Mixing Devices Key Points

1. Recommended Detention Time, Velocity Gradient & Net Power (Clause 3.1.3, Table 1)

Note: Power values based on water temperature 30°C.

2. Propeller Mixer Design (Clause 3.1.4)

• Impeller speed: 400 rpm (large) to 1400 rpm (small)
• Tank diameter to impeller diameter ratio: 5:1 to 3:1
• Tank height to diameter ratio: 1:1 to 3:1
• Blades mounted on vertical/inclined shaft producing axial flow.

3. General Notes (Clause 4.1)

• Mechanical mixers: effective, low headloss, adjustable speed.
• Require electrical energy and regular maintenance.

Formula for Power Input (P):

[ P = \mu \times G^2 \times V ]

Where:

• (P) = Power input (W)
• (\mu) = Dynamic viscosity of water (Pa·s)
• (G) = Velocity gradient (s⁻¹)
• (V) = Volume of tank (m³)

flowchart LR
    A[Tank Dimensions]
    B[Impeller Diameter]
    C[Rotation Speed (400-1400 rpm)]
    D[Velocity Gradient (G)]
    E[Power Input (P)]
    A --> B --> C --> D --> E

This summarizes key IS 709

IS 7090: Components of Mechanical Mixers - Key Specifications & Formulas

1. Mechanical Mixing Devices (Clause 3.1)

• Mixers include vane or propeller types, centrally installed in chambers.
• Shaft max unsupported length: 3 m to avoid vibration.
• Shaft speed: 60 to 100 rpm.

2. Recommended Detention Time, Velocity Gradient & Power Input (Clause 3.1.3, Table 1)

• Power calculations assume water temperature at 30°C.

3. Motor Specifications (Clause 3.2.6)

• Motor type: Totally enclosed fan-cooled (TEFC), conforming to IS 325-1978.
• Cooling method: IC 41 per IS 6362-1971.
• Protection: Minimum IP 54 enclosure per IS 4691-1985.

Additional Notes:

• Vane mixers have inlet either top or bottom; top inlet requires level adjustment to avoid water heading.
• Proper design per Table 1 ensures efficient mixing.

flowchart LR
    RawWaterInlet --> FlashMixerChamber
    FlashMixerChamber --> DeliveryFromMixer
    DeliveryFromMixer --> ChemicalDosingPipe
    ChemicalDosingPipe --> PointOfApplication
    FlashMixerChamber --> MotorDrivenPump

This layout illustrates typical jet type flash mixer flow.

Summary: Use Table 1 for power and velocity gradient selection, maintain shaft length ≤3 m, rpm 60-100, and select a TEFC motor

IS 7090: Motors and Controls - Key Formulas & Specifications

Motor Selection (Clause 3.1.2)

• Net Power Input = Power from Table 1 (see below)
• Motor Capacity = Net Power Input / (Overall Efficiency)
• Assume Overall Efficiency ≤ 75% to account for electrical & mechanical losses.

Table 1: Recommended Power Input for Mechanical Mixing Devices

Note: Power values assume water temperature at 30°C.

Motor Specifications (Clause 3.2.6)

• Motor Type: Totally enclosed fan-cooled (TEFC)
• Cooling Method: IC 41 (IS 6362-1971)
• Enclosure Protection: Minimum IP 54 (IS 4691-1985)
• Applicable for vane or propeller mixers.

Controls (Clause 3.4)

• Push button control at unit; remote control optional.
• Starters: Star-delta type, oil-immersed break type (IS 8544 Part 2-1977).
• Enclosure Protection: Minimum IP 54 (IS 2147-1562).

Motor Capacity Calculation Example:

[ \text{Motor Capacity} = \frac{\text{Net Power Input}}{0.75} ]

If Net Power Input = 288 W/m³ (for 30 sec detention), [ \text{Motor Capacity} = \frac{288}{0.75} = 384 \text{ W/m}^3 ]

flowchart TD
    A[Determine

IS 7090 - Painting of Fabricated Parts: Key Points

• Primer: Use Red Oxide (Zinc Chromate) as primer.
• Coats: Apply 1 coat of primer + at least 3 coats of finishing paint after erection (Clause 4.1).

Relevant Table: Mechanical Mixing Devices (Clause 3.1.3)

Note: Power calculations at 30°C water temperature.

Additional Specifications

• Motor for mixers: Totally enclosed fan-cooled type, IP54 protection, conforming to IS:325-1978 and IS:6362-1971 (Clause 3.2.6).
• Starter: Star-delta type as per IS:8544 (Part 2)-1977.
• Enclosures: Protection rating IP54 as per IS:2147-1562.

Summary for Painting Fabricated Parts

• Use Red Oxide Zinc Chromate primer.
• Apply 1 primer coat + minimum 3 finishing coats post-erection.
• Follow motor and starter specs for hydraulic mixing devices if applicable.

flowchart LR
    A[Fabricated Part] --> B[Apply 1 Coat Red Oxide Primer]
    B --> C[Apply 3+ Coats Finishing Paint]
    C --> D[After Erection]

This ensures corrosion protection and durability as per IS 7090.

Hydraulic Mixing as per IS 7090

Key Specifications:

1. Mechanical Rapid Mixing (Clause 3.1.1 & Table 1):
   • Detention Time: 20 to 60 seconds
   • Velocity Gradient (G): 300 to 900 sec⁻¹
   • Net Power Input: Varies with detention time and velocity gradient

Note: Power values are for water at 30°C.

2. Hydraulic Jump Mixing (Clause 5.1):

   • Velocity in flume: 3 to 3.5 m/s
   • Headloss at design flow: ≥ 300 mm
   • Rapid mixing achieved by turbulence from hydraulic jump without mechanical units.

3. Motor Specifications (Clause 3.2.6):

   • Totally enclosed fan-cooled motors (IC 41)
   • Protection: IP 54 or better
   • Conforms to IS:325-1978, IS:6362-1971, IS:4691-1985

Formula for Velocity Gradient (G):

[ G = \sqrt{\frac{P}{\mu V}} ]

• (P) = Power input (W)
• (\mu) = Dynamic viscosity (Pa·s)
• (V) = Volume of fluid (m³)

Conceptual Diagram: Hydraulic Jump Mixing

flowchart LR
    A[Coagulant Injection] --> B[Sloping Widening Flume]
    B --> C[High Velocity Flow (3-3

Hydraulic Jump (IS 7090 Key Points):

• Definition: Sudden transition in open channel flow from supercritical (high velocity, low depth) to subcritical flow (low velocity, high depth), causing turbulence and energy dissipation.
• Velocity in flume: 3 to 3.5 m/s before jump.
• Head loss: Minimum 300 mm at design flow to ensure adequate mixing.
• Purpose: Provides rapid mixing without mechanical devices by dissipating kinetic energy in a pool downstream.

Important Specifications:

• Coagulant introduced before flume.
• Flume is open, sloping, and widening.
• Turbulence from jump ensures effective mixing.

Hydraulic Jump Formulas:

Design Notes:

• Ensure flow velocity before jump is 3–3.5 m/s.
• Head loss ≥ 0.3 m for effective mixing.
• Use a sloping, widening flume to facilitate jump formation.

flowchart LR
    A[High Velocity Flow] -->|Supercritical| B[Hydraulic Jump]
    B -->|Energy Dissipation| C[Low Velocity Flow]
    C --> D[Slow Moving Pool]
    D --> E[Effective Mixing Zone]

This ensures rapid initial mixing in water treatment without mechanical mixers.

Key Specifications & Formulas for Baffled Channels (IS 7090):

• Definition (Clause 2.5): Baffled channels have fins/blades on alternate opposite walls to promote flow mixing.

• Design Velocity (Clause 5.2):
  Design channel velocity (excluding baffles) = 0.6 m/s

• Baffle Plate Material (Clause 3.2.9):
  Mild steel conforming to IS: 1730 (Part 1)-1974

• Baffle Angle & Velocity (Clause 1.5 Table 5):

  • Baffle angle with channel wall: 40° to 90°
  • Minimum velocity near baffle: 1.5 m/s

• Construction:

  • Main walls: Brick/stone masonry or RCC, finished smooth
  • Baffles: Concrete or brick, similarly finished
  • Minimum freeboard: 150 mm

Design Summary Table

Flow Around Baffle Concept (Mermaid Diagram)

flowchart LR
    A[Inflow] --> B{Baffle Plate}
    B -->|Flow splits| C[Flow around baffle end]
    C --> D[Mixing zone]
    D --> E[Outflow]

Note: Ensure smooth finishes to prevent weed growth and maintain hydraulic efficiency.

Here are the key formulas, tables, and specifications from IS 7090 for Detention Time, Velocity Gradient, and Power Input:

1. Detention Time (t)

• Time water is retained in the rapid mix unit.
• Formula:
  [ t = \frac{V}{Q} ]
  where:
  • ( V ) = volume of water in the mixer (m³)
  • ( Q ) = flow rate (m³/s)

2. Velocity Gradient (G)

• Rate of velocity change perpendicular to flow (sec⁻¹).
• From power input and viscosity:
  [ G = \sqrt{\frac{P}{\mu}} ]
  where:
  • ( P ) = power input per unit volume (W/m³)
  • ( \mu ) = absolute viscosity of water (N·s/m²)

3. Recommended Values (Table 1 from Clause 3.1.3)

Note: Power values are for water at 30°C.

Summary:

• Design detention time: 20 to 60 seconds
• Velocity gradient: 300 to 900 sec⁻¹
• Power input increases with decreasing detention time and increasing velocity gradient.

flowchart LR
    A[Water Flow Q] --> B[Rapid Mix Unit Volume V]
    B --> C[Calculate Detention Time t = V/Q]
    C --> D[Select Velocity Gradient G (300-