Understanding Trip Levels

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A trip level — also called the shutdown setpoint, emergency trip, or critical alarm — is the highest vibration or condition threshold in a machinery-protection system. When a measured value crosses it, the system automatically initiates an emergency shutdown to prevent catastrophic damage. Unlike a lower alarm level or warning level that merely notifies an operator, the trip executes a protective action on its own — it removes human decision-making from the critical path at the moment when every second counts. The trip is the last line of defence standing between a developing fault and a destroyed machine.

1. Definition: What Is a Trip Level?

A trip level is set at the vibration amplitude where continued operation risks rapid, irreversible damage to the machine or creates a safety hazard to people and plant. It is the most conservative point on a multi-tier alarm hierarchy and the only one that acts without waiting for a human. For critical turbomachinery it is mandatory under standards such as API 670, and it represents the final defence preventing failures that could destroy equipment worth millions, injure personnel, or cause an environmental release.

Because the trip is an automatic action rather than a notification, the value chosen for it is a deliberate engineering compromise. Set too low, the machine trips on harmless transients and nuisance stops erode availability and operator trust. Set too high, the protection arrives after the damage is already done. The art of trip-level setting is finding the band that catches genuine destruction early while ignoring the ordinary noise of a healthy critical machine.

2. Setting the Trip Level

Based on damage thresholds

The trip is anchored to where physical damage begins, then pulled back by a safety margin:

• Below the damage point: the setpoint must sit beneath the vibration that causes immediate mechanical harm.
• Relative to baseline: a common rule of thumb is 10–20× the machine’s healthy baseline, or the top of ISO 20816 Zone D (the old ISO 10816 Zone D), where operation is considered damaging.
• Bounded by clearances: on machines with proximity probes, the shaft-vibration trip must fire before the rotor closes the clearance and contacts a seal or the stator.
• Bounded by bearing limits: stay below the load that would fail the bearing, and factor in a sensible margin throughout.

API 670 guidance for turbomachinery

• Shaft vibration trip: typically 25 mils (635 µm) peak-to-peak, measured with proximity probes.
• Bearing housing: typically 0.5–0.6 in/s (12–15 mm/s) velocity.
• Voting: must be 2-voted — two independent sensors must agree before the trip acts.
• Time delay: typically under 1–5 seconds to confirm a sustained condition.

Machine-specific factors

• Clearances: trip before rotor contact with seals or stator.
• Bearing limits: keep the setpoint below the bearing’s load-failure threshold.
• Historical data: use the vibration recorded at previous failures of the same or sister machines.
• Manufacturer recommendations: apply OEM-specified setpoints where they are available.

3. Trip Level Versus the Other Alarms

The trip is the top rung of a staged ladder. Lower tiers buy time for planning; the trip buys nothing but survival. A typical hierarchy looks like this:

Level	Typical value	Action	Timeline
Alert	2× baseline	Investigate	Weeks to months
Warning	4× baseline	Plan maintenance	1–4 weeks
Danger	8× baseline	Urgent repair	Days
Trip	12–15× baseline	Automatic shutdown	Immediate (seconds)

The lower thresholds are the realm of condition monitoring and trend analysis, where an analyst still has the luxury of judgement. By contrast, the trip is hard-wired logic: it does not consult anyone. That is precisely why its value, voting, and delay must be engineered so carefully — there is no operator standing by to veto a bad decision.

4. Implementation Requirements

Hardware

• Permanently installed sensors — not a route-based, walk-around data collector.
• Dedicated monitoring hardware with genuine shutdown capability.
• Redundant sensors for critical trips (2-out-of-2 or 2-out-of-3 voting).
• A reliable power supply with UPS backup.
• A hard-wired shutdown path that works independently of software.

Safety-system integration

• Connection to the DCS/PLC safety system.
• Redundant trip circuits.
• A fail-safe design, so that sensor failure itself causes a trip or alarm rather than a silent loss of protection.
• Regular testing of the trip function.
• A SIL (Safety Integrity Level) rating for safety-critical applications.

Response time

• Detection to shutdown initiation: under 1 second is typical.
• Total shutdown time: depends on the equipment, from seconds to minutes.
• Fast enough to prevent damage, yet deliberate enough to avoid tripping on momentary spikes.

This protection layer is distinct from diagnostic instrumentation. A protection system answers a single yes/no question — should this machine keep running? — whereas a portable analyser answers why the vibration is rising in the first place. When a machine trips, or when its trend is creeping toward the trip band, engineers bring a portable two-channel instrument such as the Balanset-1A to the bearing housings to capture the spectrum and the 1× amplitude and phase. That diagnosis reveals whether the cause is unbalance, misalignment, or a bearing defect — and, where the root cause is unbalance, the same instrument balances the rotor in place so vibration drops well clear of the trip threshold.

5. Managing a Trip Event

When a trip occurs

1. Immediate: the equipment shuts down automatically.
2. Alarm: operators are notified of the trip condition and its cause.
3. Data capture: vibration data from before and during the trip is saved for analysis.
4. Investigation: the root cause is determined.
5. Lockout: restart is blocked until the fault is cleared.

Post-trip actions

• Inspect the equipment for damage.
• Analyse the saved vibration data.
• Identify the fault that caused the trip.
• Repair the problem.
• Verify that the trip setpoint was appropriate — neither premature nor late.
• Document the event and the lessons learned.

Trip reset

• Require a manual reset — never an automatic one.
• Confirm the cause has been addressed before clearing.
• Obtain authorisation for restart.
• Complete the post-trip inspection first.

6. Preventing False Trips

Proper setpoint selection

• High enough to avoid nuisance trips.
• Low enough to protect the equipment.
• A typical margin of 20–30% above the danger alarm.
• Allowance for the transient vibration that occurs as a machine passes through its critical speeds during startup.

Time delays

• A short delay (1–5 seconds) confirms the condition is sustained.
• It prevents trips from momentary spikes.
• Yet it must stay short enough to preserve protection.

Voting logic

• Require two sensors to agree (2-out-of-2).
• Or two of three sensors (2-out-of-3 voting).
• This prevents a single failed sensor from forcing a false trip and increases overall reliability.

7. Testing, Verification, and Standards

Functional testing and calibration

• Test the trip function periodically — annually at a minimum.
• Simulate high vibration or inject a test signal to confirm the shutdown executes.
• Test every redundant channel and document the results.
• Keep sensors and setpoints calibrated, measure the system response time, and verify every component in the trip chain.

Regulatory and standards context

• API 670: makes a vibration trip mandatory for turbomachinery above 10,000 HP and specifies setpoints, voting logic, and testing — the de-facto standard for critical equipment.
• IEC 61508: functional safety of electrical/electronic safety systems.
• IEC 61511: functional safety for the process industries.
• SIL ratings: applied to trip systems according to the risk they guard against.

In short, the trip level is the ultimate protective threshold in a machinery-monitoring system, automatically stopping equipment when vibration signals imminent catastrophic failure. Correct setpoint selection, redundant and reliable hardware, disciplined periodic testing, and tight integration with the plant safety system are what keep this last line of defence dependable — protecting both high-value rotating machinery and the people who work around it.

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