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An instrument air header schedule is the document that lists the instrument air headers in a process area and the instruments each one supplies. For each header it gives the tag, the type of header, its location, its size and the instruments connected to it, along with the route the air takes from the supply to them. The header itself, also called an air distribution manifold, is a short pipe with a row of outlets, each with its own block valve, that hands instrument air to the pneumatic devices around it: control valve positioners, valve actuators, current-to-pressure (I/P) converters and pneumatic controllers. The schedule is the written record of which outlet feeds which device, used to design the distribution and to maintain it once the plant runs.
The schedule records the header's size and type as the design set them. Air demand, line sizes and supply pressure come from the air distribution design and its calculations.
In short
- One entry per header: its tag, type, location and size, and the instruments it supplies.
- Each outlet carries the tag of the instrument it feeds, and the schedule is where that allocation is written down.
- The type describes the header: body size, outlet count and arrangement, inlet connection, outlet valves, drain and material.
- The schedule has to agree with the instrument location layout, the pneumatic hook-ups and the instrument index.
- An unused outlet is still a row, marked spare, so the room left on a header can be read off the schedule.
What the schedule records
| Field | What it holds |
|---|---|
| Header tag | The header's identifier, from the project's numbering |
| Type | The header as specified: body pipe size, number and arrangement of outlets, inlet connection, outlet valve type and size, drain connection, material |
| Location | The plant area, and the header's position on the instrument location layout |
| Size | The body and inlet size the air distribution design chose |
| Supply | The line or sub-header the header takes its air from, the start of the route to the instruments |
| Connected instruments | Each outlet with the tag of the instrument it feeds, or spare |
Practitioner references list the same core fields, the tag or number, the type, the location and the connected instruments, with the size and the supply route added in some of them. We lay the connected instruments out outlet by outlet because each outlet carries the tag of the instrument it feeds. A list of tags per header loses the one fact needed at the header itself: which valve isolates which device.
The header the schedule describes
An instrument air header is a pipe-shaped chamber with several outlets, mounted near the pneumatic instruments it supplies. The number of outlets depends on how many air-operated devices sit close by. Each outlet has its own block valve, a ball valve or a needle valve. A drain valve at the bottom lets maintenance remove the condensed water that collects there before it reaches an instrument. On the distribution piping that feeds the headers, design guides put each branch take-off on the top of a horizontal run.
A header's type can be written the way the header is ordered, and one manufacturer's ordering example shows the level of detail a complete type entry carries: a 2 inch stainless steel body with ten outlets, five on each side, a 1 inch flanged inlet, half-inch NPT lever-operated ball valves on the outlets, and a plugged ball valve on the drain. A type column at that level of detail tells two headers with different outlet counts apart on the schedule.
One header, outlet by outlet
Take AH-3401, a header on the north side of Unit 34 that supplies five air-operated valves. Its entry on the schedule reads:
| Field | AH-3401 |
|---|---|
| Type | 2 in stainless steel body, 10 outlets (5 each side), 1 in flanged inlet, 1/2 in NPT ball valve outlets, plugged ball valve drain |
| Location | Unit 34, north pipe rack, as drawn on the instrument location layout |
| Supply | The Unit 34 instrument air sub-header |
| Connected instruments | Five, on outlets 1 to 5; outlets 6 to 10 spare |
| Outlet | Feeds | Device supplied |
|---|---|---|
| 1 | FCV-302 | Positioner on the flow control valve |
| 2 | TCV-303 | Positioner on the temperature control valve |
| 3 | PCV-304 | Positioner on the pressure control valve |
| 4 | XV-301 | Actuator of the on-off valve |
| 5 | SDV-306 | Actuator of the shutdown valve |
| 6 to 10 | Spare | None |
| Drain | Not an outlet | Condensate drain at the bottom of the header |
Read across, the entry answers the questions that come up at the header: which valve to close to isolate FCV-302 for work on its positioner, which instruments lose air when AH-3401 is isolated at its inlet, and how many outlets are left for an instrument added later in the area.
Where the schedule meets the other documents
| Document | What it shares with the schedule | The mismatch to look for |
|---|---|---|
| Instrument location layout | Which header each air-operated instrument connects to, and where the header sits. On some projects the manifolds are drawn on their own drawing instead | An instrument the layout connects to a header that has no outlet on that header's entry |
| Instrument tray layout | The header positions, grouped with the junction boxes | A header drawn in one place and scheduled in another |
| Pneumatic supply hook-up | How a device is supplied from its outlet: the tubing, the filter regulator and the isolation at the header | A tag on an outlet with no pneumatic hook-up for its installation type |
| Instrument index | The tag of every instrument | An air-supplied tag in the index that sits on no outlet, or an outlet tag missing from the index |
An air-supplied instrument on no outlet is either fed some other way, which the schedule should say, or missing from it. Where the schedule and another document disagree about an outlet, the tag on that outlet in the field is the first thing to read.
Keeping it current
The schedule changes whenever an air-operated device is added, moved or taken out, and three things change with it: the row, the tag on the outlet and the location layout. An instrument moved to another header gives up its outlet on the first and takes one on the second, so both header entries change. A spare outlet allocated to a new instrument stops being spare, which is why spare outlets are rows of their own: the count left on each header can be read from the schedule without a walk through the plant.