12.3 Water & Gas Piping Schematics for Practical Stations

12.3 Water & Gas Piping Schematics for Practical Stations

Core Principle: In addition to DWV systems, the Texas State Board of Plumbing Examiners (TSBPE) requires candidates to demonstrate mastery of potable water distribution and fuel gas piping schematics. Candidates must size distribution lines using hydraulic demand tables, design code-compliant water heater safety assemblies, apply the Longest Length Method for fuel gas networks, and calculate exact cut lengths for threaded black iron and copper piping assemblies.

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1. Potable Water Supply Isometric Design & Sizing

Water supply systems must deliver adequate volume and pressure to all fixtures simultaneously while preventing hydraulic noise, water hammer, and erosive velocities.

                             [WATER SERVICE ENTRANCE]
                                        |
                       +----------------+----------------+
                       | Main Shutoff (Ball Valve)       |
                       | Water Meter                     |
                       | Backflow Preventer / PRV        |
                       +----------------+----------------+
                                        |
               +========================+========================+
               | 3/4" or 1" Main Cold Water Trunk Line           |
               |                                                 |
               +------> [3/4" Cold Supply to Water Heater]       +------> [1/2" Cold Fixture Branches]
               |                    |                                     - Water Closets
               |                    v                                     - Lavatories
               |         +----------------------+                         - Bathtubs / Showers
               |         | WATER HEATER (WH)    |                         - Kitchen Sink
               |         | - Cold Shutoff Valve |                         - Hose Bibbs
               |         | - Expansion Tank     |
               |         | - T&P Valve (150/210)|
               |         +----------+-----------+
               |                    |
               |                    v 3/4" Hot Water Main Out
               |         +=======================================+
               |         | 3/4" Hot Water Distribution Trunk     |
               |         +---------------------------------------+
               |                    |
               |                    +------> [1/2" Hot Fixture Branches]
               |                              - Lavatories
               |                              - Bathtubs / Showers
               |                              - Kitchen Sink
               |                              - Clothes Washer

Water Supply Fixture Unit (WSFU) Sizing Standards

Maximum Flow Velocity Limits

• Cold Water Piping: Maximum 8.0 feet per second (fps) to prevent excessive pressure loss and water hammer.
• Hot Water Copper Piping: Maximum 5.0 fps (especially in systems ≥ 140°F or circulating systems) to prevent cavitation and erosion-corrosion of copper tube walls.

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2. Water Heater Trim & Safety Piping Requirements

Every water heater installation depicted on an exam schematic must include the following critical components:

                  Cold Water Supply (3/4")
                             |
                        [Ball Valve] (Cold Water Shutoff Only)
                             |
                   [Thermal Expansion Tank] (On Closed Systems)
                             |
                    [Dielectric Union]
                             |
                             v
                 +=======================+
                 |                       | <--- [T&P Relief Valve (150 psi / 210°F)]
                 |      WATER HEATER     |         |
                 |      STORAGE TANK     |         | 3/4" Full-Size Rigid Discharge Line
                 |                       |         | (Pitched Downward, No Valves)
                 +=======================+         |
                             |                     v
                  [Dielectric Union]        Air Gap Discharge (6"-24" above floor/outside)
                             |
                             v
                  Hot Water Outlet (3/4")

Critical Water Heater Code Rules

1. Temperature & Pressure (T&P) Relief Valve:
   • ASME rated for 150 psi maximum pressure and 210°F maximum temperature.
   • Sensing probe must extend into the top 6 inches of the tank water volume.
2. T&P Discharge Pipe Rules (Common Exam Traps):
   • Must be full size of the valve outlet (3/4" minimum).
   • Must be rigid metal (copper, galvanized steel) or approved CPVC/PEX rated for hot water distribution.
   • Must drain by gravity (cannot slope upward or trap water).
   • Prohibited: No shutoff valves, no check valves, no reductions in size, and NO threads at the discharge terminal (to prevent anyone from capping or plugging the safety discharge line).
   • Must terminate between 6" and 24" above the floor, into an approved waste receptor, or to the building exterior directed downward.
3. Thermal Expansion Tank: Required whenever a backflow preventer, check valve, or PRV creates a closed system. Must be sized to absorb heated water volume expansion.
4. Dielectric Unions: Required at connections between dissimilar metals (e.g., copper supply lines connecting to steel water heater spuds) to prevent galvanic corrosion.

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3. Fuel Gas Piping Schematics & Longest Run Sizing

Fuel gas piping schematics (natural gas or LP/propane) are governed by the International Fuel Gas Code (IFGC) and National Fuel Gas Code (NFPA 54).

                       [GAS METER / REGULATOR]
                                  |
                                  +==========================+
                                  | Gas Main Trunk           |
                                  | (Carries Total System CFH|
                                  +=============+============+
                                                |
            +-------------------+---------------+-------------------+
            |                   |                                   |                   |
            v                   v                                   v                   v
     [FURNACE]           [WATER HEATER]                      [GAS RANGE]         [CLOTHES DRYER]
    100,000 BTU/hr        40,000 BTU/hr                       65,000 BTU/hr       35,000 BTU/hr
     (100 CFH)              (40 CFH)                            (65 CFH)            (35 CFH)
         |                      |                                   |                   |
    [Shutoff Valve]       [Shutoff Valve]                     [Shutoff Valve]     [Shutoff Valve]
         |                      |                                   |                   |
   [Sediment Trap]       [Sediment Trap]                      (Flex Connector)    (Flex Connector)

Natural Gas Properties & Demand Conversions

• Natural gas heating value: Approx. 1,000 BTU per cubic foot (CF).
• Flow Rate Formula: $\text{Demand (CFH)} = \frac{\text{Appliance Input Rating (BTU/hr)}}{1,000\text{ BTU/CF}}$
• Specific Gravity of Natural Gas: 0.60 (lighter than air).
• Standard Residential Delivery Pressure: < 0.5 psi (7" to 14" water column; nominal 0.5" w.c. pressure drop).

The Longest Length Method (Step-by-Step)

Plumbing codes require the Longest Length Method for sizing low-pressure gas piping:

1. Step 1: Determine Total Developed Length: Measure the distance from the gas meter/regulator to the most remote appliance outlet in the entire system (e.g., 60 feet to the furnace).
2. Step 2: Lock the Table Column: Find the column in the IFGC/NFPA gas sizing table that matches or exceeds this single longest developed length (60 ft). This column is used for sizing every single pipe section in the entire system.
3. Step 3: Size the Main Trunk: Sum the CFH demand of all connected appliances (100 + 40 + 65 + 35 = 240 CFH). In the 60 ft column, find the smallest pipe diameter that can carry ≥ 240 CFH.
4. Step 4: Size Each Branch: For each individual branch, use the demand of that specific appliance in the same 60 ft column.

[!WARNING] Never size individual branches using the physical branch length column! Under the Longest Length Method, every branch must be sized using the system's single longest developed run column.

Gas Pipe Capacity Reference Table (0.5 psi Natural Gas, 0.5" w.c. Drop, SG = 0.60)

Applied Example: For a total load of 240 CFH at a longest run of 60 ft, the main trunk requires a 1-inch pipe (260 CFH > 240 CFH). The 40 CFH water heater branch requires a 1/2-inch pipe (66 CFH > 40 CFH).

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4. Gas Appliance Valves & Sediment Traps (Drip Legs)

Code Rules for Gas Hardware

1. Appliance Shutoff Valve:
   • Must be located in the same room as the appliance.
   • Maximum distance: Within 6 feet of the appliance.
   • Must be upstream of the appliance union or flexible connector.
2. Sediment Trap Construction:
   • Required on appliances with automatic gas valves (water heaters, furnaces, boilers).
   • Must be installed as close to the appliance inlet as practical.
   • Must be made of a tee fitting with a capped nipple of at least 3 inches in length extending downward from the bottom outlet.
   • Flow Geometry: Gas flow must change direction by 90° through the tee so inertia forces dirt, rust, and pipe scale down into the capped drip leg rather than entering the gas valve.

         Gas Supply Line (From Shutoff)
                     |
                     v
             +---------------+ <--- Gas enters top of Tee
             |      TEE      +-----> [Gas Valve Inlet] (Flow turns 90°)
             +-------+-------+
                     |
                     | (Nipple: 3" Minimum Length)
                     v
                   [CAP] <--- Sediment Traps Debris/Moisture

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5. Threaded Black Iron Fitting Deductions

Black iron pipe used in gas practical stations requires calculating fitting allowances based on thread makeup (T_e).